Nonaqueous-electrolyte batteries and nonaqueous electrolytic solutions

ABSTRACT

The invention is to provide a nonaqueous-electrolyte battery which comprises a current collector, a positive electrode containing a lithium-containing phosphoric acid compound represented by LixMPO 4  as a positive-electrode active material, a negative electrode containing a negative-electrode active material capable of occluding and releasing lithium ions, and a nonaqueous electrolytic solution containing a chain ether and a cyclic carbonate having an unsaturated bond.

TECHNICAL FIELD

The present invention relates to nonaqueous-electrolyte batteries. Moreparticularly, the invention relates to nonaqueous-electrolyte batterieswhich employ nonaqueous electrolytic solutions that bring aboutexcellent high-output characteristics and excellent durability when ironlithium phosphate is used as the positive electrode.

BACKGROUND ART

Nonaqueous-electrolyte batteries including lithium secondary batteriesare being put to practical use in extensive applications ranging frompower sources for appliances for so-called public use, such as portabletelephones and notebook type personal computers, to vehicle-mountedpower sources for driving motor vehicles or the like. However,nonaqueous-electrolyte batteries are increasingly required to havehigher performance in recent years, and are required to attain batterycharacteristics, such as, for example, high capacity, high output,high-temperature storability, cycle characteristics, and high safety, ona high level.

In nonaqueous-electrolyte batteries, LiCoO₂ is generally used in thepositive electrodes and a carbon material capable of occluding andreleasing lithium is generally used in the negative electrodes. As thenonaqueous electrolytic solutions, use is being made of electrolyticsolutions prepared by dissolving an electrolyte salt represented byLiPF₆ in a nonaqueous organic solvent such as ethylene carbonate orethyl methyl carbonate.

Lithium cobalt oxide (LiCoO₂), which is used as a positive-electrodeactive material as shown above, has a drawback that this substance in acharged state has low thermal stability and reduces battery safety.Extensive substances have hence been investigated in search of apositive-electrode active material usable as a substitute for LiCoO₂.

As one class of substances among these, lithium-containing metal oxideshaving an olivine structure have recently received attention. Forexample, nonaqueous-electrolyte batteries employing LiFePO₄ as apositive-electrode active material can be made to have improved cyclecharacteristics and improved battery safety by taking advantage of thehigh thermal and chemical stability of LiFePO₄. In the case ofapplications such as, for example, hybrid vehicles, such properties areexceedingly useful from the standpoint of increasing the size of mountedbatteries to thereby improve energy density per unit weight or improveoutput energy density or for attaining life prolongation of batteries.

However, LiFePO₄ is known to be lower in the electronic conductivity ofinner parts of the positive-electrode active material and in high-ratedischarge characteristics as compared with LiCoO₂, LiNiO₂, LiMnO₂,Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O₂, and the like.

Furthermore, nonaqueous-electrolyte batteries employing LiFePO₄ as apositive-electrode active material have had the following problem. Whenthese batteries are repeatedly charged and discharged in ahigh-temperature environment of, for example, about 60° C., elementsincluding iron which are contained in the active material partlydissolve away with repetitions of charge/discharge, and the dissolvediron adversely affects the negative-electrode active materialconstituted of a carbon material, etc. As a result, the negativeelectrode itself is impaired in charge/discharge reversibility and otherproperties and is hence reduced in reactivity, and this tends to resultin a decrease in the capacity or output of the nonaqueous-electrolytebatteries.

Patent document 1 discloses a nonaqueous-electrolyte battery whichincludes a positive-electrode mix layer that includes apositive-electrode active material including iron lithium phosphate, aconductive material, and a binder and has a density regulated to 1.7g/cc and which further includes a nonaqueous electrolytic solutioncontaining a solvent including ethylene carbonate and a chain ether, asa nonaqueous-electrolyte battery which can have an improved dischargecapacity even during high-rate discharge in which the battery isdischarged at a relatively high current.

Patent document 2 discloses a nonaqueous-electrolyte battery whichincludes a positive-electrode active material including iron lithiumphosphate of an olivine structure as a main component, an electrolyteincluding LiPF₆ as a main component, and a nonaqueous solvent thatincludes, as a main component, a mixed solvent composed of ethylenecarbonate and diethyl carbonate or a mixed solvent composed of ethylenecarbonate and ethyl methyl carbonate and that further contains at leastvinylene carbonate and/or vinylethylene carbonate, as anonaqueous-electrolyte battery which has a high capacity and high outputand can retain the high capacity equal to the initial value even afterrepeatedly charged/discharged in a high-temperature environment of, forexample, 60° C., and which is prevented from decreasing inordinary-temperature output and low-temperature output, for example,output at around −30° C., and shows the high output equal to the initialvalue.

PRIOR-ART DOCUMENTS Patent Documents

-   Patent Document 1: JP-A-2006-236809-   Patent Document 2: JP-A-2009-4357

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

According to the techniques disclosed in patent document 1, electronicconductivity within the positive electrode is improved by improvingclose contact between the positive-electrode active material and theconductive material, between the conductive material and the currentcollector, and between the positive-electrode active material and thecurrent collector. Furthermore, by using a solvent prepared by addingdimethoxyethane, which has an exceedingly low viscosity, to ethylenecarbonate, which has a high permittivity, not only the solvent can besufficiently infiltrated into the positive-electrode mix layer but alsothe rate of movement of lithium ions is improved. These techniques arethought to improve discharge capacity during high-rate discharge inwhich the battery is discharged at a relatively high current.

However, in the case where a carbon material, which is the most generalmaterial at present, is used in the negative electrode, the batteryaccording to patent document 1 is still insufficient in durability suchas high-temperature storability and cycle characteristics because of thepoor stability of the negative-electrode coating film.

According to the techniques disclosed in patent document 2, at leastpart of the vinylene carbonate and/or vinylethylene carbonate decomposeson the electrodes to form a stable deposit, e.g., a coating film, on thesurface of the active material of the positive electrode and/or negativeelectrode. As a result, iron and other elements are inhibited fromdissolving away from the positive-electrode active material and suchdissolved elements are inhibited from adversely affecting thenegative-electrode active material. It is thought that even when thebattery is repeatedly charged/discharged, the insertion and release oflithium ions proceed smoothly and capacity deterioration and an increasein internal resistance can be inhibited to thereby inhibit the outputfrom decreasing.

However, the nonaqueous electrolytic solution has a higher viscosity andlower ionic conductivity as compared with nonaqueous-electrolyteemploying a chain ether. Because of this, when compared in initialoutput with the nonaqueous electrolytic solutions employing a chainether, this prior-art nonaqueous electrolytic solution is stillinsufficient in output in a low-temperature region, such as output atroom temperature or −30° C.

An object of the invention is to provide a nonaqueous-electrolytebattery which has a high initial output at ordinary temperature and −30°C., attains a high discharge capacity even during high-rate discharge,and has a high capacity retention after a durability test such as ahigh-temperature storage test or cycle test, and which, even after thedurability test, has the excellent initial output performance andhigh-rate discharge capacity. Another object is to provide a nonaqueouselectrolytic solution which renders the nonaqueous-electrolyte batterypossible.

Means for Solving the Problems

The present inventors diligently made investigations in order toovercome the problems described above. As a result, the inventors havefound that a nonaqueous-electrolyte battery which has a high initialoutput at ordinary temperature and −30° C., attains a high dischargecapacity even during high-rate discharge, and has a high capacityretention after a durability test such as a high-temperature storagetest or cycle test, and which, even after the durability test, has theexcellent output performance and high-rate discharge capacity equal tothe initial values can be rendered possible using a lithium-containingmetal oxide having an olivine structure as a positive-electrode activematerial, by incorporating a chain ether and either a compound havingthe function of forming a negative-electrode coating film or a compoundhaving the function of protecting the positive electrode, in a specificproportion, into an electrolytic-solution composition. The invention hasbeen thus completed.

Namely, essential points of the invention are as follows.

1. A nonaqueous-electrolyte battery which comprises: a currentcollector; a positive electrode containing a lithium-containingphosphoric acid compound represented by LixMPO₄ (wherein M is at leastone element selected from the group consisting of Group-2 to Group-12metals of the periodic table, and x satisfies 0<x≦1.2) as apositive-electrode active material; a negative electrode containing anegative-electrode active material capable of occluding and releasinglithium ions; and a nonaqueous electrolytic solution,

wherein the nonaqueous electrolytic solution contains

(1) a chain ether and(2) a cyclic carbonate having an unsaturated bond.

2. A nonaqueous-electrolyte battery which comprises: a currentcollector; a positive electrode containing a lithium-containingphosphoric acid compound represented by LixMPO₄ (wherein M is at leastone element selected from the group consisting of Group-2 to Group-12metals of the periodic table, and x satisfies 0<x≦1.2) as apositive-electrode active material; a negative electrode containing anegative-electrode active material capable of occluding and releasinglithium ions; and a nonaqueous electrolytic solution,

wherein the nonaqueous electrolytic solution contains

(1) a chain ether and(2) at least one compound selected from lithium fluorophosphates,lithium sulfonates, imide lithium salts, sulfonic acid esters, andsulfurous acid esters.

3. The nonaqueous-electrolyte battery according to 1. or 2. abovewherein the lithium-containing phosphoric acid compound is representedby LixMPO₄ (wherein M is at least one element selected from the groupconsisting of the Group-4 to Group-11 transition metals in the fourthperiod of the periodic table, and x satisfies 0<x≦1.2).

4. The nonaqueous-electrolyte battery according to 1. above wherein thecontent of the cyclic carbonate having an unsaturated bond is 0.001-5%by mass based on the whole electrolytic solution.

5. The nonaqueous-electrolyte battery according to 1. or 2. abovewherein the nonaqueous electrolytic solution contains ethylene carbonatein an amount of 10% by volume or more.

6. The nonaqueous-electrolyte battery according to 1. or 2. abovewherein the chain ether is represented by R¹OR² (wherein R¹ and R² eachrepresent a monovalent organic group which has 1-8 carbon atoms and mayhave a fluorine atom, and R¹ and R² may be the same or different).

7. The nonaqueous-electrolyte battery according to 1. or 2. abovewherein the negative-electrode active material is a carbonaceousmaterial.

8. The nonaqueous-electrolyte battery according to 1. or 2. abovewherein the current collector has an electroconductive layer on thesurface thereof, the electroconductive layer being different from thecurrent collector in compound composition.

9. A nonaqueous electrolytic solution for use in thenonaqueous-electrolyte battery according to any one of 1. to 8. above.

Effects of the Invention

According to the nonaqueous-electrolyte batteries of the invention, animprovement in high-rate discharge capacity and an increase in outputare attained in the case where a lithium-containing metal oxide havingan olivine structure is used as a positive-electrode active material, byincorporating a chain ether into a nonaqueous electrolytic solution andthereby lowering the viscosity of the nonaqueous electrolytic solutionand improving the ionic conductivity thereof. Furthermore, byincorporating, in a specific proportion, a compound having the functionof forming a negative-electrode coating film, the resistance of thecoating film on the surface of the negative electrode is prevented fromincreasing excessively, while maintaining thermal and chemicaldurability. As a result, not only high high-temperature storability andcycle characteristics can be imparted, but also an improvement inhigh-rate characteristics and an increase in output can be attained inthe battery which has undergone a durability test.

Moreover, by incorporating, in a specific proportion, a compound havingthe function of protecting the positive electrode, metal dissolutionfrom the positive-electrode active material is inhibited and theresistance of the coating film on the surface of the positive electrodeis prevented from increasing excessively, while maintaining thermal andchemical durability. As a result, not only high high-temperaturestorability and cycle characteristics can be imparted, but also animprovement in high-rate characteristics and an increase in output canbe attained in the battery which has undergone a durability test.

MODES FOR CARRYING OUT THE INVENTION

Embodiments of the invention will be explained below. However, theinvention should not be construed as being limited to the followingembodiments, and can be modified at will.

[Nonaqueous Electrolytic Solutions]

The nonaqueous electrolytic solution for use in the first aspect of theinvention contains

(1) a chain ether and(2) a cyclic carbonate having an unsaturated bond.

The nonaqueous electrolytic solution for use in the second aspect of theinvention contains

(1) a chain ether and(2) at least one compound selected from lithium fluorophosphates,lithium sulfonates, imide lithium salts, sulfonic acid esters, andsulfurous acid esters.

<Chain Ether>

The chain ether preferably is a compound represented by the generalformula R¹OR². In the formula, R¹ and R² each represent a monovalentorganic group which has 1-8 carbon atoms and may have a fluorine atom,and R¹ and R² may be the same or different.

More preferred are chain ethers having 3-10 carbon atoms.

Examples of the chain ethers having 3-10 carbon atoms include diethylether, di(2-fluoroethyl)ether, di(2,2-difluoroethyl)ether,di(2,2,2-trifluoroethyl)ether, ethyl 2-fluoroethyl ether, ethyl2,2,2-trifluoroethyl ether, ethyl 1,1,2,2-tetrafluoroethyl ether,2-fluoroethyl 2,2,2-trifluoroethyl ether, 2-fluoroethyl1,1,2,2-tetrafluoroethyl ether, 2,2,2-trifluoroethyl1,1,2,2-tetrafluoroethyl ether, ethyl n-propyl ether, ethyl3-fluoro-n-propyl ether, ethyl 3,3,3-trifluoro-n-propyl ether, ethyl2,2,3,3-tetrafluoro-n-propyl ether, ethyl 2,2,3,3,3-pentafluoro-n-propylether, 2-fluoroethyl n-propyl ether, 2-floroethyl 3-fluoro-n-propylether, 2-fluoroethyl 3,3,3-trifluoro-n-propyl ether, 2-fluoroethyl2,2,3,3-tetrafluoro-n-propyl ether, 2-fluoroethyl2,2,3,3,3-pentafluoro-n-propyl ether, 2,2,2-trifluoroethyl n-propylether, 2,2,2-trifluoroethyl 3-fluoro-n-propyl ether,2,2,2-trifluoroethyl 3,3,3-trifluoro-n-propyl ether,2,2,2-trifluoroethyl 2,2,3,3-tetrafluoro-n-propyl ether,2,2,2-trifluoroethyl 2,2,3,3,3-pentafluoro-n-propyl ether,1,1,2,2-tetrafluoroethyl n-propyl ether, 1,1,2,2-tetrafluoroethyl3-fluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl3,3,3-trifluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl2,2,3,3,3-pentafluoro-n-propyl ether, di-n-propyl ether, n-propyl3-fluoro-n-propyl ether, n-propyl 3,3,3-trifluoro-n-propyl ether,n-propyl 2,2,3,3-tetrafluoro-n-propyl ether, n-propyl2,2,3,3,3-pentafluoro-n-propyl ether, di(3-fluoro-n-propyl)ether,3-fluoro-n-propyl 3,3,3-trifluoro-n-propyl ether, 3-fluoro-n-propyl2,2,3,3-tetrafluoro-n-propyl ether, 3-fluoro-n-propyl2,2,3,3,3-pentafluoro-n-propyl ether, di(3,3,3-trifluoro-n-propyl)ether,3,3,3-trifluoro-n-propyl 2,2,3,3-tetrafluoro-n-propyl ether,3,3,3-trifluoro-n-propyl 2,2,3,3,3-pentafluoro-n-propyl ether,di(2,2,3,3-tetrafluoro-n-propyl)ether, 2,2,3,3-tetrafluoro-n-propyl2,2,3,3,3-pentafluoro-n-propyl ether,di(2,2,3,3,3-pentafluoro-n-propyl)ether, di-n-butyl ether,dimethoxymethane, methoxyethoxymethane, methoxy(2-fluoroethoxy)methane,methoxy(2,2,2-trifluoroethoxy)methane,methoxy(1,1,2,2-tetrafluoroethoxy)methane, diethoxymethane,ethoxy(2-fluoroethoxy)methane, ethoxy(2,2,2-trifluoroethoxy)methane,ethoxy(1,1,2,2-tetrafluoroethoxy)methane, di(2-fluoroethoxy)methane,2-fluoroethoxy(2,2,2-trifloroethoxy)methane,2-fluoroethoxy(1,1,2,2-tetrafluoroethoxy)methane,di(2,2,2-trifluoroethoxy)methane,2,2,2-trifluoroethoxy(1,1,2,2-tetrafluoroethoxy)methane,di(1,1,2,2-tetrafluoroethoxy)methane, dimethoxyethane,methoxyethoxyethane, methoxy(2-fluoroethoxy)ethane,methoxy(2,2,2-trifluoroethoxy)ethane,methoxy(1,1,2,2-tetrafluoroethoxy)ethane, diethoxyethane,ethoxy(2-fluoroethoxy)ethane, ethoxy(2,2,2-trifluoroethoxy)ethane,ethoxy(1,1,2,2-tetrafluoroethoxy)ethane, di(2-fluoroethoxy)ethane,2-fluoroethoxy(2,2,2-trifloroethoxy)ethane,2-fluoroethoxy(1,1,2,2-tetrafluoroethoxy)ethane,di(2,2,2-trifluoroethoxy)ethane,2,2,2-trifluoroethoxy(1,1,2,2-tetrafluoroethoxy)ethane,di(1,1,2,2-tetrafluoroethoxy)ethane, ethylene glycol di-n-propyl ether,ethylene glycol di-n-butyl ether, and diethylene glycol dimethyl ether.

Preferred of these are dimethoxymethane, diethoxymethane,ethoxymethoxymethane, ethylene glycol di-n-propyl ether, ethylene glycoldi-n-butyl ether, and diethylene glycol dimethyl ether from thestandpoints of having the high ability to solvate lithium ions andimproving dissolution into ions.

From the standpoints of having high oxidation resistance, bringing abouta high capacity retention after a durability test such as ahigh-temperature storage test or a cycle test, and enabling the batteryto have the excellent output performance and high-rate dischargecapacity equal to the initial values even after the durability test, thefollowing chain ethers are preferred of those: 2,2,2-trifluoroethyl2,2,3,3-tetrafluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl3,3,3-trifluoro-n-propyl ether, 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoro-n-propyl ether, 3,3,3-trifluoro-n-propyl2,2,3,3-tetrafluoro-n-propyl ether, 3,3,3-trifluoro-n-propyl2,2,3,3,3-pentafluoro-n-propyl ether, anddi(2,2,3,3-tetrafluoro-n-propyl)ether.

Especially preferred of those are dimethoxymethane, diethoxymethane,ethoxymethoxymethane, 2,2,2-trifluoroethyl 2,2,3,3-tetrafluoro-n-propylether, 1,1,2,2-tetrafluoroethyl 3,3,3-trifluoro-n-propyl ether, and1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoro-n-propyl ether becausethese chain ethers have low viscosity, impart high ionic conductivity,and bring about excellent battery durability.

One chain ether may be used alone, or two or more chain ethers may beused in any desired combination and proportion. The amount of the chainether to be incorporated into each nonaqueous electrolytic solution ofthe invention is desirably as follows. The concentration of the chainether in the whole nonaqueous solvent is generally 5% by volume orhigher, preferably 8% by volume or higher, more preferably 10% by volumeor higher, and is generally 70% by volume or less, preferably 60% byvolume or less, more preferably 50% by volume or less. In case where theconcentration thereof is too low, there is a tendency that it isdifficult to obtain the effect of improving ionic conductivity that isattributable to both the improvement in the degree of dissociation intolithium ions and the decrease in viscosity which are to be brought aboutby the chain ether. In case where the concentration thereof is too high,there are cases where the chain ether is inserted into the negativecarbon electrode together with lithium ions, resulting in a decrease incapacity.

The term “whole nonaqueous solvent” in this description means the wholenonaqueous electrolytic solution excluding the cyclic carbonate havingan unsaturated bond, sulfonic acid esters, sulfurous acid esters,lithium fluorophosphates, lithium sulfonates, imide lithium salts, andelectrolytes which will be described later.

<Cyclic Carbonate Having Unsaturated Bond>

In the nonaqueous electrolytic solutions of the invention, a cycliccarbonate having an unsaturated bond (hereinafter often abbreviated to“unsaturated cyclic carbonate”) can be used in order to form a coatingfilm on the surface of the negative electrode of thenonaqueous-electrolyte battery to attain battery life prolongation.

The unsaturated cyclic carbonate is not particularly limited so long asthe cyclic carbonate has a carbon-carbon double bond, and any desiredunsaturated carbonate can be used. Cyclic carbonates having an aromaticring are also included in unsaturated cyclic carbonates.

Examples of the unsaturated cyclic carbonate include vinylene carbonateand derivatives thereof, ethylene carbonates substituted with one ormore aromatic rings or substituents having a carbon-carbon double bond,phenyl carbonates, vinyl carbonates, allyl carbonates, and catecholcarbonates.

Examples of the vinylene carbonate and derivatives thereof includevinylene carbonate, methylvinylene carbonate, 4,5-dimethylvinylenecarbonate, phenylvinylene carbonate, 4,5-diphenylvinylene carbonate,vinylvinylene carbonate, 4,5-divinylvinylene carbonate, allylvinylenecarbonate, and 4,5-diallylvinylene carbonate.

Examples of the ethylene carbonates substituted with one or morearomatic rings or substituents having a carbon-carbon double bondinclude vinylethylene carbonate, 4,5-divinylethylene carbonate,4-methyl-5-vinylethylene carbonate, 4-allyl-5-vinylethylene carbonate,phenylethylene carbonate, 4,5-diphenylethylene carbonate,4-phenyl-5-vinylethylene carbonate, 4-allyl-5-phenylethylene carbonate,allylethylene carbonate, 4,5-diallylethylene carbonate, and4-methyl-5-allylethylene carbonate.

Especially preferred unsaturated cyclic carbonates among these arevinylene carbonate, methylvinylene carbonate, 4,5-dimethylvinylenecarbonate, vinylvinylene carbonate, 4,5-divinylvinylene carbonate,allylvinylene carbonate, 4,5-diallylvinylene carbonate, vinylethylenecarbonate, 4,5-divinylethylene carbonate, 4-methyl-5-vinylethylenecarbonate, allylethylene carbonate, 4,5-diallylethylene carbonate,4-methyl-5-allylethylene carbonate, and 4-allyl-5-vinylethylenecarbonate. These carbonates are more suitable because the carbonatesform a stable interface-protective coating film.

The molecular weight of the unsaturated cyclic carbonate is notparticularly limited, and the carbonate may have any desired molecularweight unless the effects of the invention are considerably lessenedthereby. The molecular weight thereof is preferably 50-250. So long asthe unsaturated cyclic carbonate has a molecular weight within thatrange, it is easy to ensure the solubility of the unsaturated cycliccarbonate in the nonaqueous electrolytic solution and the effects of theinvention are apt to be sufficiently produced. The molecular weight ofthe unsaturated cyclic carbonate is more preferably 80 or higher, and ismore preferably 150 or lower. Methods for producing the unsaturatedcyclic carbonate are not particularly limited, and the carbonate can beproduced by a known method selected at will.

One unsaturated cyclic carbonate may be used alone, or two or moreunsaturated cyclic carbonates may be used in any desired combination andproportion. The amount of the unsaturated cyclic carbonate to beincorporated is not particularly limited, and the carbonate may beincorporated in any desired amount unless the effects of the inventionare considerably lessened thereby. The amount of the unsaturated cycliccarbonate to be incorporated per 100% by mass the nonaqueous solvent,i.e., the amount thereof based on the whole nonaqueous electrolyticsolution, is preferably 0.001% by mass or more, more preferably 0.01% bymass or more, even more preferably 0.1% by mass or more, and ispreferably 5% by mass or less, more preferably 4% by mass or less, evenmore preferably 3% by mass or less. So long as the amount of theunsaturated cyclic carbonate is within that range, it is easy to producethe effect of sufficiently improving the cycle characteristics of thenonaqueous-electrolyte battery. In addition, it is easy to avoid thetrouble that the battery has reduced high-temperature storability andevolves a gas in an increased amount, resulting in a decrease indischarge capacity retention.

The cyclic carbonate having an unsaturated bond may have a fluorineatom. The number of fluorine atoms possessed by the unsaturated cycliccarbonate having a fluorine atom (hereinafter often abbreviated to“fluorinated unsaturated cyclic carbonate”) is not particularly limitedso long as the number thereof is 1 or more. In particular, the number ofthe fluorine atoms is generally 6 or less, preferably 4 or less. Mostpreferred are fluorinated unsaturated cyclic carbonates having one ortwo fluorine atoms.

Examples of the fluorinated unsaturated cyclic carbonate includefluorinated vinylene carbonate derivatives and fluorinated ethylenecarbonate derivatives substituted with one or more aromatic rings orsubstituents having a carbon-carbon double bond.

Examples of the fluorinated vinylene carbonate derivatives include4-fluorovinylene carbonate, 4-fluoro-5-methylvinylene carbonate,4-fluoro-5-phenylvinylene carbonate, 4-allyl-5-fluorovinylene carbonate,and 4-fluoro-5-vinylvinylene carbonate.

Examples of the fluorinated ethylene carbonate derivatives substitutedwith one or more aromatic rings or substituents having a carbon-carbondouble bond include 4-fluoro-4-vinylethylene carbonate,4-fluoro-4-allylethylene carbonate, 4-fluoro-5-vinylethylene carbonate,4-fluoro-5-allylethylene carbonate, 4,4-difluoro-4-vinylethylenecarbonate, 4,4-difluoro-4-allylethylene carbonate,4,5-difluoro-4-vinylethylene carbonate, 4,5-difluoro-4-allylethylenecarbonate,

-   4-fluoro-4,5-divinylethylene carbonate, 4-fluoro-4,5-diallylethylene    carbonate, 4,5-difluoro-4,5-divinylethylene carbonate,    4,5-difluoro-4,5-diallylethylene carbonate,    4-fluoro-4-phenylethylene carbonate, 4-fluoro-5-phenylethylene    carbonate, 4,4-difluoro-5-phenylethylene carbonate, and    4,5-difluoro-4-phenylethylene carbonate.

Especially preferred fluorinated unsaturated cyclic carbonates amongthese are 4-fluorovinylene carbonate, 4-fluoro-5-methylvinylenecarbonate, 4-fluoro-5-vinylvinylene carbonate, 4-allyl-5-fluorovinylenecarbonate, 4-fluoro-4-vinylethylene carbonate, 4-fluoro-4-allylethylenecarbonate, 4-fluoro-5-vinylethylene carbonate, 4-fluoro-5-allylethylenecarbonate, 4,4-difluoro-4-vinylethylene carbonate,4,4-difluoro-4-allylethylene carbonate, 4,5-difluoro-4-vinylethylenecarbonate, 4,5-difluoro-4-allylethylene carbonate,4-fluoro-4,5-divinylethylene carbonate, 4-fluoro-4,5-diallylethylenecarbonate, 4,5-difluoro-4,5-divinylethylene carbonate, and4,5-difluoro-4,5-diallylethylene carbonate. These carbonates are moresuitable because the carbonates form a stable interface-protectivecoating film.

The molecular weight of the fluorinated unsaturated cyclic carbonate isnot particularly limited, and the carbonate may have any desiredmolecular weight unless the effects of the invention are considerablylessened thereby. The molecular weight thereof is preferably 50-250. Solong as the fluorinated unsaturated cyclic carbonate has a molecularweight within that range, it is easy to ensure the solubility of thefluorinated cyclic carbonate in the nonaqueous electrolytic solution andthe effects of the invention are apt to be produced. Methods forproducing the fluorinated unsaturated cyclic carbonate are notparticularly limited, and the carbonate can be produced by a knownmethod selected at will. The molecular weight of the fluorinatedunsaturated cyclic carbonate is more preferably 100 or higher, and ismore preferably 200 or lower.

One fluorinated unsaturated cyclic carbonate may be used alone, or twoor more fluorinated unsaturated cyclic carbonates may be used in anydesired combination and proportion. The amount of the fluorinatedunsaturated cyclic carbonate to be incorporated is not particularlylimited, and the carbonate may be incorporated in any desired amountunless the effects of the invention are considerably lessened thereby.The amount of the fluorinated unsaturated cyclic carbonate to beincorporated per 100% by mass the nonaqueous electrolytic solution isusually preferably 0.01% by mass or more, more preferably 0.1% by massor more, even more preferably 0.2% by mass or more, and is preferably 5%by mass or less, more preferably 4% by mass or less, even morepreferably 3% by mass or less. So long as the amount of the fluorinatedunsaturated cyclic carbonate is within that range, it is easy to producethe effect of sufficiently improving the cycle characteristics of thenonaqueous-electrolyte battery. In addition, it is easy to avoid thetrouble that the battery has reduced high-temperature storability andevolves a gas in an increased amount, resulting in a decrease indischarge capacity retention.

<Sulfonic Acid Esters>

Sulfonic acid esters can be used in the nonaqueous electrolyticsolutions to be used in the invention, in order to attain battery lifeprolongation. Examples of the sulfonic acid esters include cyclicsulfonic acid esters having 3-6 carbon atoms and chain sulfonic acidesters having 1-4 carbon atoms.

Examples of the cyclic sulfonic acid esters having 3-6 carbon atomsinclude 1,3-propanesultone, 1-methyl-1,3-propanesultone,2-methyl-1,3-propanesultone, 3-methyl-1,3-propanesultone,1-ethyl-1,3-propanesultone, 2-ethyl-1,3-propanesultone,3-ethyl-1,3-propanesultone, 1-fluoro-1,3-propanesultone,2-fluoro-1,3-propanesultone, 3-fluoro-1,3-propanesultone,1,4-butanesultone, 1-methyl-1,4-butanesultone,2-methyl-1,4-butanesultone, 3-methyl-1,4-butanesultone,4-methyl-1,4-butanesultone, 1-ethyl-1,4-butanesultone,2-ethyl-1,4-butanesultone, 3-ethyl-1,4-butanesultone,4-ethyl-1,4-butanesultone, 1-propene-1,3-sultone,1-fluoro-1-propene-1,3-sultone, 2-fluoro-1-propene-1,3-sultone,3-fluoro-1-propene-1,3-sultone, 1,4-butanesultone, 1-butene-1,4-sultone,and 3-butene-1,4-sultone.

More suitable of these are 1,3-propanesultone,1-methyl-1,3-propanesultone, 2-methyl-1,3-propanesultone,3-methyl-1,3-propanesultone, 1-ethyl-1,3-propanesultone,1,4-butanesultone, 1-methyl-1,4-butanesultone,2-methyl-1,4-butanesultone, 3-methyl-1,4-butanesultone,4-methyl-1,4-butanesultone, 1-propene-1,3-sultone, and the like becausesuch cyclic sulfonic acid esters have the high ability to protect theelectrode interface based on interaction with the electrode surface andimprove storability and cycle durability.

Examples of the chain sulfonic acid esters having 1-4 carbon atomsinclude methyl fluorosulfonate, ethyl fluorosulfonate, propylfluorosulfonate, methyl methanesulfonate, ethyl methanesulfonate, propylmethanesulfonate, methyl ethanesulfonate, ethyl ethanesulfonate, methylvinylsulfonate, and ethyl vinylsulfonate.

More suitable of these are methyl fluorosulfonate, ethylfluorosulfonate, methyl methanesulfonate, ethyl methanesulfonate, methylethanesulfonate, ethyl ethanesulfonate, methyl vinylsulfonate, ethylvinylsulfonate, and the like because such chain sulfonic acid estershave the high ability to protect the electrode interface based oninteraction with the electrode surface and improve storability and cycledurability.

One sulfonic acid ester may be used alone, or two or more sulfonic acidesters may be used in any desired combination and proportion. The amountof the sulfonic acid ester to be incorporated is not particularlylimited, and the sulfonic acid ester may be incorporated in any desiredamount unless the effects of the invention are considerably lessenedthereby. The amount of the sulfonic acid ester to be incorporated per100% by mass the nonaqueous electrolytic solution usually is preferably0.01% by mass or more, more preferably 0.1% by mass or more, even morepreferably 0.2% by mass or more, and is preferably 5% by mass or less,more preferably 4% by mass or less, even more preferably 3% by mass orless. So long as the amount of the sulfonic acid ester is within thatrange, it is easy to produce the effect of sufficiently improving thecycle characteristics of the nonaqueous-electrolyte battery. Inaddition, it is easy to avoid the trouble that the battery has reducedhigh-temperature storability and evolves a gas in an increased amount,resulting in a decrease in discharge capacity retention.

<Sulfurous Acid Esters>

Cyclic sulfurous acid esters can be used in the nonaqueous electrolyticsolutions to be used in the invention, in order to attain battery lifeprolongation. Examples of the sulfurous acid esters include cyclicsulfurous acid esters having 3-6 carbon atoms.

Examples of the cyclic sulfurous acid esters having 3-6 carbon atomsinclude ethylene sulfite, 4-methylethylene sulfite, 4,4-dimethylethylenesulfite, 4,5-dimethylethylene sulfite, 4-ethylethyene sulfite,4,4-diethylethylene sulfite, and 4,5-diethylethylene sulfite.

More suitable of these are ethylene sulfite and 4-methylethylene sulfitebecause these cyclic sulfurous acid esters have the high ability toprotect the electrode interface based on interaction with the electrodesurface and improve storability and cycle durability.

One sulfurous acid ester may be used alone, or two or more sulfurousacid esters may be used in any desired combination and proportion. Theamount of the sulfurous acid ester to be incorporated is notparticularly limited, and the sulfurous acid ester may be incorporatedin any desired amount unless the effects of the invention areconsiderably lessened thereby. The amount of the cyclic sulfurous acidester to be incorporated per 100% by mass the nonaqueous electrolyticsolution usually is preferably 0.01% by mass or more, more preferably0.1% by mass or more, even more preferably 0.2% by mass or more, and ispreferably 5% by mass or less, more preferably 4% by mass or less, evenmore preferably 3% by mass or less. So long as the amount of the cyclicsulfurous acid ester is within that range, it is easy to produce theeffect of sufficiently improving the cycle characteristics of thenonaqueous-electrolyte battery. In addition, it is easy to avoid thetrouble that the battery has reduced high-temperature storability andevolves a gas in an increased amount, resulting in a decrease indischarge capacity retention.

<Other Ingredients>

In the invention, it is possible to use cyclic carbonates includingethylene carbonate, chain carbonates, cyclic and chain esters other thancarbonic acid esters, cyclic ethers, sulfone compounds, and the like.

<Cyclic Carbonates>

Ethylene Carbonate

It is preferred that the nonaqueous electrolytic solutions to be used inthe invention each should contain ethylene carbonate, and the contentthereof based on the whole nonaqueous solvent is as follows. The lowerlimit thereof is preferably 10% by volume or more, and the upper limitthereof is preferably 70% by volume or less.

Cyclic Carbonates Other than Ethylene Carbonate

Examples of the cyclic carbonates other than ethylene carbonate includecyclic carbonates having an alkylene group with 3 or 4 carbon atoms.

Specifically, examples of the cyclic carbonates having an alkylene groupwith 3 or 4 carbon atoms include propylene carbonate and butylenecarbonate. Especially preferred of these is propylene carbonate from thestandpoint of improving battery characteristics on the basis of animprovement in the degree of dissociation into lithium ions.

It is desirable in the invention that one or more such cyclic carbonatesother than ethylene carbonate should be incorporated in a concentrationof generally 5% by volume or higher, preferably 10% by volume or higher,based on the whole nonaqueous solvent in the nonaqueous electrolyticsolution. In case where the concentration thereof is less than the lowerlimit, the incorporation thereof brings about little increase in theelectrical conductivity of the nonaqueous electrolytic solution of theinvention. In particular, there are cases where the incorporationthereof does not contribute to an improvement in the high-currentdischarge characteristics of the nonaqueous-electrolyte battery of theinvention. It is also desirable that one or more cyclic carbonates otherthan ethylene carbonate should be incorporated in a concentration ofgenerally 40% by volume or less, preferably 35% by volume or less. Incase where the concentration thereof exceeds the range, there is atendency that the nonaqueous electrolytic solution has an increasedviscosity coefficient and this reduces the electrical conductivitythereof. In particular, there are cases where the nonaqueous-electrolytebattery is reduced in high-current discharge characteristics.

With respect to the term “whole nonaqueous solvent” used here also, thisterm means the whole nonaqueous electrolytic solution excluding thecyclic carbonate having an unsaturated bond, sulfonic acid esters, andsulfurous acid esters which were described above, the lithiumfluorophosphates, lithium sulfonates, and imide lithium salts which willbe described later, and the electrolytes which will be described later,as in the case of ethylene carbonate.

Saturated Cyclic Carbonates Having Fluorine Atom(s)

Saturated cyclic carbonates having a fluorine atom (hereinafter oftenabbreviated to “fluorinated saturated cyclic carbonates”) are notparticularly limited. Examples thereof include derivatives of saturatedcyclic carbonates having an alkylene group with 2-6 carbon atoms.Specific examples thereof include ethylene carbonate derivatives.Examples of the ethylene carbonate derivatives include products offluorination of either ethylene carbonate or ethylene carbonatesubstituted with one or more alkyl groups (e.g., alkyl groups having 1-4carbon atoms). Preferred of these are such fluorination products having1-8 fluorine atoms.

Specific examples thereof include monofluoroethylene carbonate,4,4-difluoroethylene carbonate, 4,5-difluoroethylene carbonate,4-fluoro-4-methylethylene carbonate, 4,5-difluoro-4-methylethylenecarbonate, 4-fluoro-5-methylethylene carbonate,4,4-difluoro-5-methylethylene carbonate, 4-(fluoromethyl)ethylenecarbonate, 4-(difluoromethyl)ethylene carbonate,4-(trifluoromethyl)ethylene carbonate, 4-(fluoromethyl)-4-fluoroethylenecarbonate, 4-(fluoromethyl)-5-fluoroethylene carbonate,4-fluoro-4,5-dimethylethylene carbonate,4,5-difluoro-4,5-dimethylethylene carbonate, and4,4-difluoro-5,5-dimethylethylene carbonate.

More preferred of these is at least one member selected from the groupconsisting of monofluoroethylene carbonate, 4,4-difluoroethylenecarbonate, 4,5-difluoroethylene carbonate, and4,5-difluoro-4,5-dimethylethylene carbonate because these carbonatesimpart high ionic conductivity and satisfactorily form aninterface-protective coating film.

One fluorinated saturated cyclic carbonate may be used alone, or two ormore fluorinated saturated cyclic carbonates may be used in any desiredcombination and proportion. The amount of the fluorinated saturatedcyclic carbonate to be incorporated is not particularly limited, and thecarbonate may be used in any desired amount unless the effects of theinvention are considerably lessened thereby. However, the amount thereofper 100% by mass the nonaqueous solvent is preferably 0.01% by mass ormore, more preferably 0.1% by mass or more, even more preferably 0.2% bymass or more. With respect to the upper limit thereof, the amount of thecarbonate is less than 50% by mass, preferably 45% by mass or less. Solong as the amount of the fluorinated saturated cyclic carbonate iswithin that range, it is easy to produce the effect of sufficientlyimproving the cycle characteristics of the nonaqueous-electrolytebattery. In addition, it is easy to avoid the trouble that the batteryhas reduced high-temperature storability and evolves a gas in anincreased amount, resulting in a decrease in discharge capacityretention.

<Chain Carbonates>

Chain carbonates having 3-7 carbon atoms are preferred.

Examples of the chain carbonates having 3-7 carbon atoms includedimethyl carbonate, diethyl carbonate, di-n-propyl carbonate,diisopropyl carbonate, n-propyl isopropyl carbonate, ethyl methylcarbonate, methyl n-propyl carbonate, n-butyl methyl carbonate, isobutylmethyl carbonate, t-butyl methyl carbonate, ethyl n-propyl carbonate,n-butyl ethyl carbonate, isobutyl ethyl carbonate, and t-butyl ethylcarbonate.

Preferred of these are dimethyl carbonate, diethyl carbonate,di-n-propyl carbonate, diisopropyl carbonate, n-propyl isopropylcarbonate, ethyl methyl carbonate, and methyl n-propyl carbonate.

Especially preferred of these are dimethyl carbonate, diethyl carbonate,and ethyl methyl carbonate.

Chain carbonates having a fluorine atom (hereinafter, the carbonates areoften abbreviated to “fluorinated chain carbonates”) also are suitable.The number of fluorine atoms possessed by each fluorinated chaincarbonate also is not particularly limited so long as the number thereofis 1 or more. However, the number thereof is generally 6 or less,preferably 4 or less. In the case where a fluorinated chain carbonatehas a plurality of fluorine atoms, these fluorine atoms may be the sameor different. Examples of the fluorinated chain carbonates includeethylene carbonate derivatives, dimethyl carbonate derivatives, ethylmethyl carbonate derivatives, and diethyl carbonate derivatives.

Examples of the dimethyl carbonate derivatives include fluoromethylmethyl carbonate, difluoromethyl methyl carbonate, trifluoromethylmethyl carbonate, bis(fluoromethyl) carbonate, bis(difluoro)methylcarbonate, and bis(trifluoro)methyl carbonate.

Examples of the ethyl methyl carbonate derivatives include 2-fluoroethylmethyl carbonate, ethyl fluoromethyl carbonate, 2,2-difluoroethyl methylcarbonate, 2-fluoroethyl fluoromethyl carbonate, ethyl difluoromethylcarbonate, 2,2,2-trifluoroethyl methyl carbonate, 2,2-difluoroethylfluoromethyl carbonate, 2-fluoroethyl difluoromethyl carbonate, andethyl trifluoromethyl carbonate.

Examples of the diethyl carbonate derivatives include ethyl2-fluoroethyl carbonate, ethyl 2,2-difluoroethyl carbonate,bis(2-fluoroethyl) carbonate, ethyl 2,2,2-trifluoroethyl carbonate,2,2-difluoroethyl 2′-fluoroethyl carbonate, bis(2,2-difluoroethyl)carbonate, 2,2,2-trifloroethyl 2′-fluoroethyl carbonate,2,2,2-trifluoroethyl 2′,2′-difluoroethyl carbonate, andbis(2,2,2-trifluoroethyl) carbonate.

With respect to the chain carbonates explained above also, any one ofthese chain carbonates may be incorporated alone into each nonaqueouselectrolytic solution of the invention or two or more thereof may beincorporated in any desired combination and proportion.

In each nonaqueous electrolytic solution to be used in the invention, itis desirable that at least one chain carbonate should be incorporated ina concentration of preferably 15% by volume or higher, more preferably20% by volume or higher, even more preferably 25% by volume or higher,based on the whole nonaqueous solvent in the nonaqueous electrolyticsolution. It is also desirable that the chain carbonate be incorporatedin a concentration of 85% by volume or less, more preferably 80% byvolume or less, even more preferably 75% by volume or less.

So long as the concentration thereof is within that range, thenonaqueous electrolytic solution suffers neither an increase inviscosity nor a decrease in ionic conductivity and, hence, hassatisfactory high-current electrical conduction characteristics.

In each nonaqueous electrolytic solution of the invention, ethylenecarbonate and a specific chain carbonate may be incorporated in specificamounts together with a specific chain ether compound. Thus, theperformance of the electrolytic solution can be greatly improved.

For example, in the case where dimethoxyethane was selected as the chainether, it is preferred to select ethyl methyl carbonate as the specificchain carbonate. In this case, it is especially preferred that ethylenecarbonate should be incorporated in an amount of 15% by volume to 40% byvolume, dimethoxyethane be incorporated in an amount of 10% by volume to40% by volume, and ethyl methyl carbonate be incorporated in an amountof 30% by volume to 60% by volume. By selecting ethyl methyl carbonateas the chain carbonate and by selecting these incorporation amounts, thecompatibility temperature range can be widened and thelower-temperature-side precipitation temperature for lithium salts canbe lowered.

In the case where diethoxyethane was selected as the chain ether inanother example, it is preferred to select dimethyl carbonate as thespecific chain carbonate. In this case, it is especially preferred thatethylene carbonate should be incorporated in an amount of 15% by volumeto 40% by volume, diethoxyethane be incorporated in an amount of 20% byvolume to 60% by volume, and dimethyl carbonate be incorporated in anamount of 15% by volume to 60% by volume. By selecting dimethylcarbonate as the chain carbonate and by selecting these incorporationamounts, the nonaqueous electrolytic solution can be made to have areduced viscosity and improved ionic conductivity while lowering thelower-temperature-side precipitation temperature for lithium salts, andhigh output can be obtained even at low temperatures.

<Cyclic Esters other than Cyclic Carbonic Acid Esters>

Examples of the cyclic esters other than cyclic carbonic acid estersinclude cyclic esters having 3-12 carbon atoms.

Specific examples thereof include γ-butyrolactone, γ-valerolactone,γ-caprolactone, and ε-caprolactone. Of these, γ-butyrolactone isespecially preferred from the standpoint of the improvement in batterycharacteristics which is attributable to an improvement in the degree ofdissociation into lithium ions.

It is desirable in the invention that such a cyclic ester should beincorporated in a concentration of generally 5% by volume or higher,preferably 10% by volume or higher, based on the whole nonaqueoussolvent in the nonaqueous electrolytic solution. In case where theconcentration thereof is lower than the lower limit, the effect ofincreasing the electrical conductivity of the nonaqueous electrolyticsolution of the invention is low and, in particular, there are caseswhere the incorporation of the cyclic ester does not contribute to animprovement in the high-current discharge characteristics of thenonaqueous-electrolyte battery. It is also desirable that the cyclicester should be incorporated in a concentration of generally 40% byvolume or less, preferably 35% by volume or less. In case where theconcentration thereof exceeds the range, there is a tendency that thenonaqueous electrolytic solution has an increased viscosity coefficientand this reduces the electrical conductivity thereof or increases theresistance of the negative electrode. In particular, there are caseswhere the nonaqueous-electrolyte battery has reduced high-currentdischarge characteristics.

<Chain Esters Other than Chain Carbonic Acid Esters>

Examples of the chain esters other than chain carbonic acid estersinclude chain esters having an alkylene group with 3-7 carbon atoms.

Specific examples thereof include methyl acetate, ethyl acetate,n-propyl acetate, isopropyl acetate, n-butyl acetate, isobutyl acetate,t-butyl acetate, methyl propionate, ethyl propionate, n-propylpropionate, isopropyl propionate, n-butyl propionate, isobutylpropionate, t-butyl propionate, methyl butyrate, ethyl butyrate,n-propyl butyrate, isopropyl butyrate, methyl isobutyrate, ethylisobutyrate, n-propyl isobutyrate, and isopropyl isobutyrate.

Especially preferred of these are methyl acetate, ethyl acetate,n-propyl acetate, n-butyl acetate, methyl propionate, ethyl propionate,n-propyl propionate, isopropyl propionate, methyl butyrate, ethylbutyrate, and the like from the standpoint of improving ionicconductivity on the basis of a decrease in viscosity.

It is desirable in the invention that such a chain ester should beincorporated in a concentration of generally 10% by volume or higher,preferably 15% by volume or higher, based on the whole nonaqueoussolvent in the nonaqueous electrolytic solution. In case where theconcentration thereof is lower than the lower limit, the effect ofincreasing the electrical conductivity of the nonaqueous electrolyticsolution of the invention is low and, in particular, there are caseswhere the incorporation of the chain ester does not contribute to animprovement in the high-current discharge characteristics of thenonaqueous-electrolyte battery. It is also desirable that the chainester should be incorporated in a concentration of generally 60% byvolume or less, preferably 50% by volume or less. In case where theconcentration thereof exceeds the range, there is a tendency that thenegative electrode has increased resistance and thenonaqueous-electrolyte battery suffers a decrease in high-currentdischarge characteristics and a decrease in cycle characteristics.

<Cyclic Ethers>

Examples of the cyclic ethers include cyclic ethers having an alkylenegroup with 3-6 carbon atoms.

Specific examples thereof include tetrahydrofuran,2-methyltetrahydrofuran, 3-methyltetrahydrofuran, 1,3-dioxane,2-methyl-1,3-dioxane, 4-methyl-1,3-dioxane, 1,4-dioxane, and compoundsformed by fluorinating these.

Especially preferred of these are 2-methyltetrahydrofuran and2-methyl-1,3-dioxane. This is because these cyclic ethers have lowviscosity and the high ability to solvate lithium ions and, hence,improve dissociation into ions, thereby imparting high ionicconductivity.

The amount of the cyclic ether to be incorporated, per 100% by volumethe nonaqueous solvent, usually is preferably 5% by volume or more, morepreferably 10% by volume or more, even more preferably 15% by volume ormore, and is preferably 40% by volume or less, more preferably 35% byvolume or less, even more preferably 30% by volume or less. So long asthe amount thereof is within that range, it is easy to ensure the effectof improving ionic conductivity which is attributable to the improvementin the degree of dissociation into lithium ions and the decrease inviscosity which are brought about by the cyclic ether. In addition, inthe case where the negative-electrode active material is a carbonaceousmaterial, it is easy to avoid the trouble that the cyclic ether isinserted into the active material together with lithium ions, resultingin a decrease in capacity.

<Sulfone Compounds>

Preferred sulfone compounds are cyclic sulfones having 3-6 carbon atomsand chain sulfones having 2-6 carbon atoms. It is preferred that thenumber of sulfonyl groups per molecule should be 1 or 2.

Examples of the cyclic sulfones include monosulfone compounds such astrimethylene sulfone compounds, tetramethylene sulfone compounds, andhexamethylene sulfone compounds and disulfone compounds such astrimethylene disulfone compounds, tetramethylene disulfone compounds,and hexamethylene disulfone compounds. From the standpoints ofpermittivity and viscosity, tetramethylene sulfone compounds,tetramethylene disulfone compounds, hexamethylene sulfone compounds, andhexamethylene disulfone compounds are more preferred of those, andtetramethylene sulfone compounds (sulfolane compounds) are especiallypreferred.

The sulfolane compounds preferably are sulfolane and/or sulfolanederivatives (hereinafter, these compounds including sulfolane are oftenreferred to simply as “sulfolane compounds”). The sulfolane derivativespreferably are sulfolane compounds in which one or more of the hydrogenatoms bonded to the carbon atoms constituting the sulfolane ring eachhave been replaced with a fluorine atom or an alkyl group.

Preferred of these are 2-methylsulfolane, 3-methylsulfolane,2-fluorosulfolane, 3-fluorosulfolane, 2,2-difluorosulfolane,2,3-difluorosulfolane, 2,4-difluorosulfolane, 2,5-difluorosulfolane,3,4-difluorosulfolane, 2-fluoro-3-methylsulfolane,2-fluoro-2-methylsulfolane, 3-fluoro-3-methylsulfolane,3-fluoro-2-methylsulfolane, 4-fluoro-3-methylsulfolane,4-fluoro-2-methylsulfolane, 5-fluoro-3-methylsulfolane,5-fluoro-2-methylsulfolane, 2-fluoromethylsulfolane,3-fluoromethylsulfolane, 2-difluoromethylsulfolane,3-difluoromethylsulfolane, 2-trifluoromethylsulfolane,3-trifluoromethylsulfolane, 2-fluoro-3-(trifluoromethyl)sulfolane,3-fluoro-3-(trifluoromethyl)sulfolane,4-fluoro-3-(trifluoromethyl)sulfolane, and5-fluoro-3-(trifluoromethyl)sulfolane, from the standpoint that thesesulfolane compounds have high ionic conductivity and bring about highinput/output characteristics.

Examples of the chain sulfones include dimethyl sulfone, ethyl methylsulfone, diethyl sulfone, n-propyl methyl sulfone, n-propyl ethylsulfone, di-n-propyl sulfone, isopropyl methyl sulfone, isopropyl ethylsulfone, diisopropyl sulfone, n-butyl methyl sulfone, n-butyl ethylsulfone, t-butyl methyl sulfone, t-butyl ethyl sulfone, monofluoromethylmethyl sulfone, difluoromethyl methyl sulfone, trifluoromethyl methylsulfone, monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone,trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone, ethylmonofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyltrifluoromethyl sulfone, perfluoroethyl methyl sulfone, ethyltrifluoroethyl sulfone, ethyl pentafluoroethyl sulfone,di(trifluoroethyl) sulfone, perfluorodiethyl sulfone, fluoromethyln-propyl sulfone, difluoromethyl n-propyl sulfone, trifluoromethyln-propyl sulfone, fluoromethyl isopropyl sulfone, difluoromethylisopropyl sulfone, trifluoromethyl isopropyl sulfone, trifluoroethyln-propyl sulfone, trifluoroethyl isopropyl sulfone, pentafluoroethyln-propyl sulfone, pentafluoroethyl isopropyl sulfone, trifluoroethyln-butyl sulfone, trifluoroethyl t-butyl sulfone, pentafluoroethyln-butyl sulfone, and pentafluoroethyl t-butyl sulfone.

Preferred of these are dimethyl sulfone, ethyl methyl sulfone, diethylsulfone, n-propyl methyl sulfone, isopropyl methyl sulfone, n-butylmethyl sulfone, t-butyl methyl sulfone, monofluoromethyl methyl sulfone,difluoromethyl methyl sulfone, trifluoromethyl methyl sulfone,monofluoroethyl methyl sulfone, difluoroethyl methyl sulfone,trifluoroethyl methyl sulfone, pentafluoroethyl methyl sulfone, ethylmonofluoromethyl sulfone, ethyl difluoromethyl sulfone, ethyltrifluoromethyl sulfone, ethyl trifluoroethyl sulfone, ethylpentafluoroethyl sulfone, trifluoromethyl n-propyl sulfone,trifluoromethyl isopropyl sulfone, trifluoroethyl n-butyl sulfone,trifluoroethyl t-butyl sulfone, trifluoromethyl n-butyl sulfone,trifluoromethyl t-butyl sulfone, and the like, from the standpoint thatthese sulfone compounds have high ionic conductivity and bring abouthigh input/output characteristics.

The amount of the sulfone compound to be incorporated, per 100% by massthe nonaqueous solvent, usually is preferably 0.3% by mass or more, morepreferably 0.5% by mass or more, even more preferably 1% by mass ormore, and is preferably 40% by mass or less, more preferably 35% by massor less, even more preferably 30% by mass or less. So long as the amountthereof is within that range, it is easy to obtain the effect ofimproving durability such as cycle characteristics and storability. Inaddition, the viscosity of the nonaqueous electrolytic solution can beregulated so as to be within a proper range, and a decrease inelectrical conductivity can be avoided. Furthermore, it is easy to avoidthe trouble that the nonaqueous-electrolyte battery decreases incharge/discharge capacity retention when charged and discharged at ahigh current density.

<Electrolytes>

Examples of electrolytes which can be contained in the nonaqueouselectrolytic solutions to be used in the invention include lithiumfluorophosphates, lithium sulfonates, and imide lithium salts. Preferredof these lithium salts are compounds having the high ability to beadsorbed onto or interact with the surface of the positive-electrodeactive material. In the case where a compound having the high ability tobe adsorbed onto or interact with the electrode surface is used, theresistance of the coating film on the electrode surface can be preventedfrom increasing excessively, while maintaining thermal and chemicaldurability. As a result, not only high high-temperature storability andcycle characteristics can be imparted, but also an improvement inhigh-rate characteristics and an increase in output can be attained inthe battery which has undergone a durability test. Besides those lithiumsalts, any desired lithium salts can be used.

<Lithium Fluorophosphates>

Examples of the lithium fluorophosphates include lithium fluorophosphateand lithium difluorophosphate. Such lithium salts may be used incombination. In particular, lithium difluorophosphate is preferredbecause this lithium salt has the high ability to be adsorbed onto orinteract with the surface of the electrode active material.

<Lithium Sulfonates>

Examples of the lithium sulfonates include lithium methanesulfonate,lithium monofluoromethanesulfonate, lithium difluoromethanesulfonate,and lithium trifluoromethanesulfonate. Such lithium salts may be used incombination. In particular, lithium trifluoromethanesulfonate ispreferred because this lithium salt has the high ability to be adsorbedonto or interact with the surface of the electrode active material.

<Imide Lithium Salts>

Examples of the imide lithium salts include LiN(FCO₂)₂, LiN(FCO)(FSO₂),LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, the lithiumsalt of cyclic 1,2-perfluoroethanedisulfonylimide, the lithium salt ofcyclic 1,3-perfluoropropanedisulfonylimide, and LiN(CF₃SO₂)(C₄F₉SO₂).Such salts may be used in combination. In particular, LiN(FCO)(FSO₂),LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, and thelithium salt of cyclic 1,2-perfluoroethanedisulfonylimide are preferredbecause these lithium salts have the high ability to be adsorbed onto orinteract with the surface of the electrode active material.

<Other Lithium Salts>

Examples of lithium salts other than the lithium fluorophosphates,lithium sulfonates, and imide lithium salts include:

carboxylic acid lithium salts such as lithium formate, lithium acetate,lithium monofluoroacetate, lithium difluoroacetate, and lithiumtrifluoroacetate;

lithium methide compounds such as LiC(FSO₂)₃, LiC(CF₃SO₂)₃, andLiC(C₂F₅SO₂)₃;

lithium oxalatoborate salts such as lithium difluorooxalatoborate andlithium bis(oxalato)borate;

lithium oxalatophosphate salts such as lithiumtetrafluorooxalatophosphate, lithium difluorooxalatophosphate, andlithium tris(oxalato)phosphate; and other fluorine-containingorganolithium salts such as LiBF₄, LiPF₄(CF₃)₂, LiPF₄(C₂F₅)₂,LiPF₄(CF₃SO₂)², LiPF₄(C₂F₅SO₂)₂, LiBF₂(CF₃)₂, LiBF₂(C₂F₅)₂,LiBF₂(CF₃SO₂)₂, and LiBF₂(C₂F₅SO₂)₂.

Preferred of these, for use as main electrolytes for the nonaqueouselectrolytic solutions, are LiPF₆, LiBF₄, lithiumtrifluoromethanesulfonate, LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, the lithium salt of cyclic1,2-perfluoroethanedisulfonylimide, and the lithium salt of cyclic1,3-perfluoropropanedisulfonylimide from the standpoint of improvingbattery performance.

The concentration of each of these main electrolytes in the nonaqueouselectrolytic solutions is not particularly limited. However, theconcentration thereof is generally 0.5 mol/L or higher, preferably 0.6mol/L or higher, more preferably 0.7 mol/L or higher, and is generally 3mol/L or less, preferably 2 mol/L or less, more preferably 1.8 mol/L orless, especially preferably 1.5 mol/L or less. In the case where theconcentration of the electrolyte added is within that range, the effectof improving battery characteristics is sufficiently produced and it iseasy to avoid the trouble that the resistance of charge movementincreases to reduce charge/discharge performance.

One of those main electrolytes for the nonaqueous electrolytic solutionsmay be used alone, or two or more thereof may be used in combination. Inthe case where two or more main electrolytes are used in combination, apreferred example is a combination of LiPF₆ with LiBF₄, LiN(FSO₂)₂,LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, the lithium salt ofcyclic 1,2-perfluoroethanedisulfonylimide, the lithium salt of cyclic1,3-perfluoropropanedisulfonylimide, or the like. This combination hasthe effect of improving output characteristics, high-ratecharge/discharge characteristics, high-temperature storability, cyclecharacteristics, etc.

Also in the case where two or more main electrolytes are contained in anonaqueous electrolytic solution, the concentration of the electrolytesis not particularly limited. However, the total concentration of themain electrolytes is generally 0.5 mol/L or higher, preferably 0.6 mol/Lor higher, more preferably 0.7 mol/L or higher, and is generally 3 mol/Lor less, preferably 2 mol/L or less, more preferably 1.8 mol/L or less,especially preferably 1.5 mol/L or less. In the case where theconcentration of the electrolytes added is within that range, the effectof improving battery characteristics is sufficiently produced and it iseasy to avoid the trouble that the resistance of charge amount increasesto reduce charge/discharge performance.

It is also preferred that an electrolyte other than the mainelectrolytes for the nonaqueous electrolytic solutions should be addedbesides one or more of the main electrolytes. Preferred examples ofelectrolytes to be added when LiPF₆ is used as a main electrolyteinclude LiBF₄, lithium monofluorophosphate, lithium difluorophosphate,lithium formate, lithium acetate, lithium monofluoroacetate, lithiumdifluoroacetate, lithium trifluoroacetate, lithium methanesulfonate,lithium monofluoromethanesulfonate, lithium difluoromethanesulfonate,lithium trifluoromethanesulfonate, LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂),lithium difluorooxalatoborate, lithium tetrafluorooxalatophosphate, andlithium difluorooxalatophosphate. Addition of these electrolytes has theeffect of improving output characteristics and high-ratecharacteristics.

Furthermore, it is preferred to add lithium fluorophosphate, lithiumdifluorophosphate, lithium trifluoromethanesulfonate, LiN(FSO₂)₂,LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, the lithium salt ofcyclic 1,2-perfluoroethanedisulfonylimide, the lithium salt of cyclic1,3-perfluoropropanedisulfonylimide, lithium difluorooxalatoborate,lithium tetrafluorooxalatophosphate, lithium difluorooxalatophosphate,or the like besides the main electrolyte, because the addition thereofhas the effect of improving high-temperature storability and cyclecharacteristics.

The concentration of the electrolyte added to a nonaqueous electrolyticsolution besides the main electrolyte is also not particularly limited.However, the concentration thereof is preferably 0.01% by mass orhigher, more preferably 0.03% by mass or higher, even more preferably0.05% by mass or higher, and is preferably 8% by mass or less, morepreferably 6% by mass or less, even more preferably 5% by mass or less.When the concentration of the electrolyte added is within that range,the effect of improving battery characteristics is sufficiently producedand it is easy to avoid the trouble that the resistance of chargemovement increases to reduce charge/discharge performance.

Incidentally, LiBF₄, LiN(FSO₂)₂, LiN(FSO₂)(CF₃SO₂), LiN(CF₃SO₂)₂,LiN(C₂F₅SO₂)₂, the lithium salt of cyclic1,2-perfluoroethanedisulfonylimide, the lithium salt of cyclic1,3-perfluoropropanedisulfonylimide, and the like have the effect ofimproving the performance of the battery regardless of whether thesesalts are added as main electrolytes or added as electrolytes besides amain electrolyte.

<Overcharge Inhibitor>

An overcharge inhibitor can be used in the nonaqueous electrolyticsolutions of the invention in order to effectively inhibit thenonaqueous-electrolyte batteries from bursting or firing when broughtinto an overcharged state or the like.

Examples of the overcharge inhibitor include: aromatic compounds such asbiphenyl, alkylbiphenyls, terphenyl, partly hydrogenated terphenyls,cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, anddibenzofuran; products of partial fluorination of these aromaticcompounds, such as 2-fluorobiphenyl, o-cyclohexylfluorobenzene, andp-cyclohexylfluorobenzene; and fluorine-containing anisole compoundssuch as 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole,and 3,5-difluoroanisole. Preferred of these are aromatic compounds suchas biphenyl, alkylbiphenyls, terphenyl, partly hydrogenated terphenyls,cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, anddibenzofuran. One of these may be used alone, or two or more thereof maybe used in combination. In the case where two or more compounds are usedin combination, the following combinations are especially preferred fromthe standpoint of a balance between overcharge-preventive properties andhigh-temperature storability: a combination of cyclohexylbenzene witht-butylbenzene or t-amylbenzene; and a combination of at least onemember selected from aromatic compounds containing no oxygen, such asbiphenyl, alkylbiphenyls, terphenyl, partly hydrogenated terphenyls,cyclohexylbenzene, t-butylbenzene, and t-amylbenzene, with at least onemember selected from oxygen-containing aromatic compounds such asdiphenyl ether and dibenzofuran.

The amount of the overcharge inhibitor to be incorporated is notparticularly limited, and the overcharge inhibitor may be incorporatedin any desired amount unless the effects of the invention areconsiderably lessened thereby. The amount of the overcharge inhibitor ispreferably 0.01-5% by mass per 100% by mass the nonaqueous solvent. Solong as the amount thereof is within that range, it is easy tosufficiently produce the effect of the overcharge inhibitor and it iseasy to avoid the trouble that battery characteristics includinghigh-temperature storability decrease. The amount of the overchargeinhibitor is more preferably 0.01% by mass or more, even more preferably0.1% by mass or more, especially preferably 0.2% by mass or more, and ismore preferably 3% by mass or less, even more preferably 2% by mass orless.

<Other Aids>

Other known aids can be used in the nonaqueous electrolytic solutions ofthe invention. Examples of the other aids include: carbonate compoundssuch as erythritane carbonate, spiro-bis-dimethylene carbonate, andmethoxyethyl methyl carbonate; carboxylic acid anhydrides such assuccinic anhydride, glutaric anhydride, maleic anhydride, citraconicanhydride, glutaconic anhydride, itaconic anhydride, diglycolicanhydride, cyclohexanedicarboxylic anhydride,cyclopentanetetracarboxylic dianhydride, and phenylsuccinic anhydride;Spiro compounds such as 2,4,8,10-tetraoxaspiro[5.5]undecane and3,9-divinyl-2,4,8,10-tetraoxaspiro[5.5]undecane; sulfur-containingcompounds such as busulfan, sulfolene, diphenyl sulfone,N,N-dimethylmethanesulfonamide, and N,N-diethylmethanesulfonamide;nitrogen-containing compounds such as 1-methyl-2-pyrrolidinone,1-methyl-2-piperidone, 3-methyl-2-oxazolidinone,1,3-dimethyl-2-imidazolidinone, and N-methylsuccinimide; hydrocarboncompounds such as heptane, octane, nonane, decane, and cycloheptane; andfluorine-containing aromatic compounds such as fluorobenzene,difluorobenzene, hexafluorobenzene, and benzotrifluoride. One of theseaids may be used alone, or two or more thereof may be used incombination. By adding these aids, capacity retentivity afterhigh-temperature storage and cycle characteristics can be improved.

The amount of the other aids to be incorporated is not particularlylimited, and the other aids may be incorporated in any desired amountunless the effects of the invention are considerably lessened thereby.The amount of the other aids is preferably 0.01-5% by mass per 100% bymass the nonaqueous solvent. So long as the amount thereof is withinthat range, it is easy to sufficiently produce the effects of the otheraids and it is easy to avoid the trouble that battery characteristicsincluding high-load discharge characteristics decrease. The amount ofthe other aids to be incorporated is more preferably 0.1% by mass ormore, even more preferably 0.2% by mass or more, and is more preferably3% by mass or less, even more preferably 1% by mass or less.

The nonaqueous electrolytic solutions described above include thenonaqueous electrolytic solutions present in inner parts of thenonaqueous-electrolyte batteries according to the invention.Specifically, the invention includes: the nonaqueous electrolyticsolution present in a nonaqueous-electrolyte battery obtained byseparately synthesizing constituent elements for a nonaqueouselectrolytic solution, such as a lithium salt, a solvent, and aids,preparing the nonaqueous electrolytic solution from the substantiallyseparate constituent elements, and introducing the nonaqueouselectrolytic solution into a battery separately assembled by the methodwhich will be described later. The invention further includes: the casein which constituent elements for a nonaqueous electrolytic solution ofthe invention are separately introduced into a battery and mixedtogether within the battery to thereby obtain the same composition asthe nonaqueous electrolytic solution of the invention; and the case inwhich a compound serving as a component of a nonaqueous electrolyticsolution of the invention is generated within the nonaqueous-electrolytebattery to obtain the same composition as the nonaqueous electrolyticsolution of the invention.

[Battery Configuration]

The nonaqueous-electrolyte batteries of the invention may have the samebattery configuration as conventionally known nonaqueous-electrolytebatteries. Usually, the batteries of the invention have a configurationobtained by superposing a positive electrode and a negative electrodethrough a porous film (separator) impregnated with a nonaqueouselectrolytic solution of the invention and disposing the stack in a case(outer case). Consequently, the shapes of the nonaqueous-electrolytebatteries of the invention are not particularly limited, and may be anyof cylindrical, prismatic, laminate type, coin type, large-size, andother shapes.

[Negative Electrode]

The negative-electrode active material to be used in the negativeelectrode is described below. The negative-electrode active material isnot particularly limited so long as the active material is capable ofelectrochemically occluding and releasing lithium ions. Examples thereofinclude a carbonaceous material, an alloy material, and alithium-containing composite metal oxide material.

[Negative-Electrode Active Material]

Examples of the negative-electrode active material include acarbonaceous material, an alloy material, and a lithium-containingcomposite metal oxide material.

The carbonaceous material to be used as a negative-electrode activematerial preferably is a material selected from:

(1) natural graphites;(2) carbonaceous materials obtained by subjecting artificialcarbonaceous substances and artificial graphitic substances to a heattreatment at a temperature in the range of 400-3,200° C. one or moretimes;(3) carbonaceous materials giving a negative-electrode active-materiallayer which is composed of at least two carbonaceous substancesdiffering in crystallinity and/or has an interface where at least twocarbonaceous substances differing in crystallinity are in contact witheach other; and(4) carbonaceous materials giving a negative-electrode active-materiallayer which is composed of at least two carbonaceous substancesdiffering in orientation and/or has an interface where at least twocarbonaceous substances differing in orientation are in contact witheach other.

This is because this carbonaceous material brings about a satisfactorybalance between initial irreversible capacity and high-current-densitycharge/discharge characteristics. One of the carbonaceous materials (1)to (4) may be used alone, or two or more thereof may be used in anydesired combination and proportion.

Examples of the artificial carbonaceous substances and artificialgraphitic substances in (2) above include natural graphites, coal coke,petroleum coke, coal pitch, petroleum pitch, carbonaceous substancesobtained by oxidizing these pitches, needle coke, pitch coke, carbonmaterials obtained by partly graphitizing these cokes, products of thepyrolysis of organic substances, such as furnace black, acetylene black,and pitch-derived carbon fibers, organic substances capable ofcarbonization and products of the carbonization thereof, or solutionsobtained by dissolving any of such organic substances capable ofcarbonization in a low-molecular organic solvent, e.g., benzene,toluene, xylene, quinoline, or n-hexane, and products of thecarbonization of these solutions.

The alloy material to be used as a negative-electrode active material isnot particularly limited so long as the material is capable of occludingand releasing lithium. Use may be made of elemental lithium, anelemental metal or alloy which forms a lithium alloy, or any ofcompounds thereof, such as oxides, carbides, nitrides, silicides,sulfides, and phosphides. The elemental metal or alloy which forms alithium alloy preferably is a material including any of the metals andsemimetals in Group 13 and Group 14 (that is, carbon is excluded). Morepreferred are elemental aluminum, silicon, and tin (hereinafter, thesemetals are often referred to as “specific metallic elements”) and alloysor compounds containing one or more atoms of any of these metals. One ofsuch materials may be used alone, or two or more thereof may be used inany desired combination and proportion.

Examples of the negative-electrode active material including atoms of atleast one member selected from the specific metallic elements include:the elemental metal which is any one of the specific metallic elements;alloys constituted of two or more specific metallic elements; alloysconstituted of one or more specific metallic elements and one or moreother metallic elements; compounds containing one or more specificmetallic elements; and composite compounds, e.g., oxides, carbides,nitrides, silicides, sulfides, or phosphides, of these compounds. Byusing any of these elemental metals, alloys, and metal compounds as anegative-electrode active material, a battery having a higher capacitycan be obtained.

Examples of the negative-electrode active material further includecompounds formed by the complicated bonding of any of those compositecompounds to one or more elemental metals or alloys or to severalelements, e.g., nonmetallic elements. Specifically, in the case ofsilicon and tin, for example, use can be made of an alloy of thoseelements with a metal which does not function as a negative electrode.In the case of tin, for example, use may be made of a complicatedcompound constituted of a combination of five to six elements includingtin, a metal which functions as a negative electrode and is not silicon,a metal which does not function as a negative electrode, and anonmetallic element.

Preferred of those negative-electrode active materials are the elementalmetal which is any one of the specific metallic elements, alloys of twoor more of the specific metallic elements, and oxides, carbides,nitrides, and other compounds of the specific metallic elements. This isbecause these negative-electrode active materials give a battery havinga high capacity per unit mass. Especially preferred are the elementalmetal(s), alloys, oxides, carbides, nitrides, and the like of siliconand/or tin from the standpoints of capacity per unit mass andenvironmental burden.

The lithium-containing composite metal oxide material to be used as anegative-electrode active material is not particularly limited so longas the material is capable of occluding and releasing lithium. However,from the standpoint of high-current-density charge/dischargecharacteristics, materials containing both titanium and lithium arepreferred, and lithium-containing composite metal oxide materialscontaining titanium are more preferred. Even more preferred arecomposite oxides of lithium and titanium (hereinafter abbreviated to“lithium-titanium composite oxides”). Namely, use of a lithium-titaniumcomposite oxide having a spinel structure is especially preferredbecause incorporation of this composite oxide into a negative-electrodeactive material for nonaqueous-electrolyte batteries is effective inconsiderably reducing output resistance.

Also preferred are lithium-titanium composite oxides in which thelithium or titanium has been replaced by one or more other metallicelements, e.g., at least one element selected from the group consistingof Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, and Nb.

Such metal oxide preferably is a lithium-titanium composite oxiderepresented by general formula (1) wherein 0.7≦x≦1.5, 1.5≦y≦2.3, and0≦z≦1.6, because the structure thereof is stable during lithium iondoping/undoping.

Li_(x)Ti_(y)M_(z)O₄  (1)

[In general formula (1), M represents at least one element selected fromthe group consisting of Na, K, Co, Al, Fe, Ti, Mg, Cr, Ga, Cu, Zn, andNb.]

Of the compositions represented by general formula (I), structuresrepresented by general formula (1) wherein

1.2≦x≦1.4,1.5≦y≦1.7, and z=0  (a)

0.9≦x≦1.1,1.9≦y≦2.1, and z=0 or  (b)

0.7≦x≦0.9,2.1≦y≦2.3, and z=0  (c)

are especially preferred because these structures bring about asatisfactory balance among battery performances.

Especially preferred representative compositions of those compounds are:Li_(4/3)Ti_(5/3)O₄ for (a), Li₁Ti₂O₄ for (b), and Li_(4/5)Ti_(11/5)O₄for (c). Preferred examples of the structure wherein z≠0 includeLi_(4/3)Ti_(4/3)Al_(1/3)O₄.

<Properties of Carbonaceous Material>

In the case where a carbonaceous material is used as anegative-electrode active material, it is desirable that thecarbonaceous material should have the following properties.

(X-Ray Parameter)

The carbonaceous material preferably has a value of d (interplanarspacing) for the lattice planes (002), as determined by X-raydiffractometry in accordance with the method of the Japan Society forPromotion of Scientific Research, of 0.335 nm or larger. The value of dthereof is generally 0.360 nm or less, preferably 0.350 nm or less, morepreferably 0.345 nm or less. The crystallite size (Lc) of thecarbonaceous material, as determined by X-ray diffractometry inaccordance with the method of the Japan Society for Promotion ofScientific Research, is preferably 1.0 nm or larger, more preferably 1.5nm or larger.

(Volume-Average Particle Diameter)

The volume-average particle diameter of the carbonaceous material, interms of volume-average particle diameter (median diameter) asdetermined by the laser diffraction/scattering method, is generally 1 μmor more, preferably 3 μm or more, more preferably 5 μm or more,especially preferably 7 μm or more, and is generally 100 μm or less,preferably 50 μm or less, more preferably 40 μm or less, even morepreferably 30 μm or less, especially preferably 25 μm or less.

When the volume-average particle diameter thereof is less than the lowerlimit of that range, there are cases where irreversible capacityincreases, leading to a loss in initial battery capacity. When thevolume-average particle diameter thereof exceeds the upper limit of thatrange, there are cases where such a carbonaceous material is undesirablefrom the standpoint of battery production because an uneven coatingsurface is apt to result when an electrode is produced through coatingfluid application.

Volume-average particle diameter is determined by dispersing the carbonpowder in a 0.2% by mass aqueous solution (about 10 mL) ofpoly(oxyethylene (degree of polymerization, 20)) sorbitan monolaurate asa surfactant and examining the dispersion with a laserdiffraction/scattering type particle size distribution analyzer (LA-700,manufactured by HORIBA, Ltd.). The median diameter determined throughthis measurement is defined as the volume-average particle diameter ofthe carbonaceous material in the invention.

(Raman R Value, Raman Half-Value Width)

The Raman R value of the carbonaceous material as determined by theargon ion laser Raman spectroscopy is generally 0.01 or higher,preferably 0.03 or higher, more preferably 0.1 or higher, and isgenerally 1.5 or lower, preferably 1.2 or lower, more preferably 1 orlower, especially preferably 0.5 or lower.

When the Raman R value thereof is within that range, it is easy to avoidthe trouble that the surface of the particles has too high crystallinityand the number of intercalation sites into which lithium comes withcharge/discharge decreases, resulting in a decrease in suitability forcharge. In addition, it is possible to prevent the trouble that when acoating fluid containing a carbonaceous material is applied to a currentcollector and the resultant coating is pressed to heighten the densityof the negative electrode, then the crystals are apt to orient indirections parallel to the electrode plate and this leads to a decreasein load characteristics. Furthermore, it is easy to avoid the troublethat the surface of the particles has reduced crystallinity and enhancedreactivity with the nonaqueous electrolytic solution and this leads to adecrease in efficiency and enhanced gas evolution.

The Raman half-value width around 1,580 cm⁻¹ of the carbonaceousmaterial is not particularly limited. However, the half-value widththereof is generally 10 cm⁻¹ or more, preferably 15 cm⁻¹ or more, and isgenerally 100 cm⁻¹ or less, preferably 80 cm⁻¹ or less, more preferably60 cm⁻¹ or less, especially preferably 40 cm⁻¹ or less.

When the Raman half-value width thereof is within that range, it is easyto avoid the trouble that the surface of the particles has too highcrystallinity and the number of intercalation sites into which lithiumcomes with charge/discharge decreases, resulting in a decrease insuitability for charge. In addition, it is possible to prevent thetrouble that when a coating fluid containing a carbonaceous material isapplied to a current collector and the resultant coating is pressed toheighten the density of the negative electrode, then the crystals areapt to orient in directions parallel to the electrode plate and thisleads to a decrease in load characteristics. Furthermore, it is easy toavoid the trouble that the surface of the particles has reducedcrystallinity and enhanced reactivity with the nonaqueous electrolyticsolution and this leads to a decrease in efficiency and enhanced gasevolution.

The examination for a Raman spectrum is made with a Raman spectrometer(Raman spectrometer manufactured by Japan Spectroscopic Co., Ltd.). Inthe examination, a sample is charged into a measuring cell by causingthe sample to fall naturally into the cell and the surface of the samplein the cell is irradiated with argon ion laser light while rotating thecell in a plane perpendicular to the laser light. The Raman spectrumobtained is examined for the intensity I_(A) of a peak PA around 1,580cm⁻¹ and the intensity I_(g) of a peak PB around 1,360 cm⁻¹. The ratiobetween these intensities R(R═I_(B)/I_(A)) is calculated. The Raman Rvalue calculated through this examination is defined as the Raman Rvalue of the carbonaceous material in the invention. Furthermore, thehalf-value width of the peak P_(A) around 1,580 cm⁻¹ in the Ramanspectrum obtained is measured, and this value is defined as the Ramanhalf-value width of the carbonaceous material in the invention.

Conditions for the Raman spectroscopy are as follows.

Wavelength of argon ion laser: 514.5 nm

Laser power on sample: 15-25 mW

Resolution: 10-20 cm⁻¹

Examination range: 1,100 cm⁻¹ to 1,730 cm⁻¹

Analysis for Raman R value and Raman half-value width: backgroundprocessing

Smoothing: simple average; convolution, 5 points

(4) BET Specific Surface Area

The BET specific surface area of the carbonaceous material, in terms ofthe value of specific surface area as determined by the BET method, isgenerally 0.1 m²·g⁻¹ or larger, preferably 0.7 m²·g⁻¹ or larger, morepreferably 1.0 m²·g⁻¹ or larger, especially preferably 1.5 m²·g⁻¹ orlarger, and is generally 100 m²·g⁻¹ or smaller, preferably 25 m²·g⁻¹ orsmaller, more preferably 15 m²·g⁻¹ or smaller, especially preferably 10m²·g⁻¹ or smaller.

When the BET specific surface area thereof is within that range, thiscarbonaceous material, when used as a negative-electrode material,readily accepts lithium during charge and inhibits lithium depositionfrom occurring on the electrode surface. Furthermore, when thiscarbonaceous material is used as a negative-electrode material, thereactivity thereof with the nonaqueous electrolytic solution is not sohigh and, hence, gas evolution is slight. A preferred battery thereforeis easy to obtain.

The determination of specific surface area by the BET method is madewith a surface area meter (a fully automatic surface area measuringapparatus manufactured by Ohkura Riken Co., Ltd.) by preliminarilydrying a sample at 350° C. for 15 minutes in a nitrogen stream and thenmeasuring the specific surface area thereof by the gas-flowing nitrogenadsorption BET one-point method using a nitrogen/helium mixture gasprecisely regulated so as to have a nitrogen pressure of 0.3 relative toatmospheric pressure. The specific surface area determined through thismeasurement is defined as the BET specific surface area of thecarbonaceous material in the invention.

(Roundness)

When the carbonaceous material is examined for roundness as an index tothe degree of sphericity thereof, the roundness thereof is preferablywithin the range shown below. Roundness is defined by “Roundness=(lengthof periphery of equivalent circle having the same area as projectedparticle shape)/(actual length of periphery of projected particleshape)”. When a particle has a roundness of 1, this particletheoretically is a true sphere.

The closer to 1 the roundness of carbonaceous-material particles havinga particle diameter in the range of 3-40 μm, the more the particles aredesirable. The roundness of the particles is desirably 0.1 or higher,preferably 0.5 or higher, more preferably 0.8 or higher, even morepreferably 0.85 or higher, especially preferably 0.9 or higher. Thehigher the roundness, the more the high-current-density charge/dischargecharacteristics are improved. Consequently, when carbonaceous-materialparticles have a roundness within that range, the negative-electrodeactive material has improved suitability for loading and retains lowinterparticle resistance. Consequently, short-time high-current-densitycharge/discharge characteristics are less apt to decrease.

Roundness is determined with a flow type particle image analyzer (FPIA,manufactured by Sysmex Industrial Corp.). About 0.2 g of a sample isdispersed in a 0.2% by mass aqueous solution (about 50 mL) ofpoly(oxyethylene(20)) sorbitan monolaurate as a surfactant, and anultrasonic wave of 28 kHz is propagated to the dispersion for 1 minuteat an output of 60 W. Thereafter, particles having a particle diameterin the range of 3-40 μm are examined with the analyzer having adetection range set at 0.6-400 μm. The roundness determined through thismeasurement is defined as the roundness of the carbonaceous material inthe invention.

Methods for improving roundness are not particularly limited. However, acarbonaceous material in which the particles have been rounded by arounding treatment is preferred because this material gives an electrodein which the interstices among particles are uniform in shape. Examplesof the rounding treatment include: a method in which shear force orcompressive force is applied to thereby mechanically make the shape ofthe particles close to sphere; and a method of mechanical/physicaltreatment in which fine particles are aggregated into particles by meansof the bonding force of a binder or of the fine particles themselves.

(Tap Density)

The tap density of the carbonaceous material is generally 0.1 g·cm⁻³ orhigher, preferably 0.5 g·cm⁻³ or higher, more preferably 0.7 g·cm⁻³ orhigher, especially preferably 1 g·cm⁻³ or higher, and is preferably 2.2g·cm⁻³ or less, more preferably 2.1 g·cm⁻³ or less, especiallypreferably 2.0 g·cm⁻³ or less. In the case where the tap density thereofis within that range, this carbonaceous material, when used in anegative electrode, can attain an increase in loading density to rendera high-capacity battery easy to obtain. Furthermore, the amount ofinterparticle interstices in the electrode is not excessively small and,hence, electrical conductivity among the particles is ensured. Thus, itis easy to obtain preferred battery characteristics.

Tap density is determined by dropping a sample through a sieve having anopening size of 300 μm into a 20-cm³ tapping cell to fill the cell withthe sample up to the brim, subsequently conducting a tapping operation1,000 times over a stroke length of 10 mm using a powder densimeter(e.g., Tap Denser, manufactured by Seishin Enterprise Co., Ltd.), andcalculating the tap density from the resultant volume of the sample andthe weight thereof. The tap density calculated through this measurementis defined as the tap density of the carbonaceous material in theinvention.

(Orientation Ratio)

The orientation ratio of the carbonaceous material is generally 0.005 orgreater, preferably 0.01 or greater, more preferably 0.015 or greater,and is generally 0.67 or less. When the orientation ratio thereof iswithin that range, it is easy to avoid the trouble that high-densitycharge/discharge characteristics decrease. The upper limit of that rangeis a theoretical upper limit of the orientation ratio of carbonaceousmaterials.

Orientation ratio is determined by X-ray diffractometry after a sampleis molded by compaction. A molded object obtained by packing 0.47 g of asample into a molding machine having a diameter of 17 mm and compactingthe sample at 58.8 MN·m⁻² is set with clay on a sample holder forexamination so as to be flush with the holder. This sample is examinedfor X-ray diffraction. From the intensities of the resultant (110)diffraction peak and (004) diffraction peak for the carbon, the ratiorepresented by (110) diffraction peak intensity/(004) diffraction peakintensity is calculated. The orientation ratio calculated through thismeasurement is defined as the orientation ratio of the carbonaceousmaterial in the invention.

Conditions for the X-ray diffractometry are as follows. Incidentally,“20” represents diffraction angle.

Target: Cu (Kα line) graphite monochromator

Slits:

-   -   Divergence slit=0.5 degrees    -   Receiving slit=0.15 mm    -   Scattering slit=0.5 degrees

Examination range and step angle/measuring time:

(110) plane: 75° ≦ 2θ ≦ 80° 1°/60 sec (004) plane: 52° ≦ 2θ ≦ 57° 1°/60sec

(Aspect Ratio (Powder))

The aspect ratio of the carbonaceous material is generally 1 or greater,and is generally 10 or less, preferably 8 or less, more preferably 5 orless. When the aspect ratio thereof is within that range, it is easy toavoid the trouble that the carbonaceous material causes streak lines inelectrode plate formation and an even coating surface cannot beobtained, resulting in a decrease in high-current-densitycharge/discharge characteristics. Incidentally, the lower limit of thatrange is a theoretical lower limit of the aspect ratio of carbonaceousmaterials.

In determining aspect ratio, particles of the carbonaceous material areexamined with a scanning electron microscope with enlargement. Fifty arearbitrarily selected from graphite particles fixed to an edge face of ametal having a thickness of 50 μm or smaller, and each particle isexamined in a three-dimensional manner while rotating and inclining thestage to which the sample is fixed. In this examination, the length ofthe longest axis A of each carbonaceous-material particle and the lengthof the shortest axis B perpendicular to that axis are measured, and theaverage of the A/B values is determined. The aspect ratio (A/B)determined through this measurement is defined as the aspect ratio ofthe carbonaceous material in the invention.

<Configuration of Negative Electrode and Method of Production thereof.

Any known method can be used for electrode production unless thisconsiderably lessens the effects of the invention. For example, a binderand a solvent are added to a negative-electrode active materialoptionally together with a thickener, conductive material, filler, etc.to obtain a slurry and this slurry is applied to a current collector anddried. Thereafter, the coated current collector is pressed. Thus, anelectrode can be formed.

In the case where an alloy material is employed, use may be made of amethod in which a thin-film layer containing the negative-electrodeactive material described above (negative-electrode active-materiallayer) is formed by a technique such as vapor deposition, sputtering, orplating.

(Current Collector)

As the current collector for holding the negative-electrode activematerial, a known current collector can be used at will. Examples of thecurrent collector for the negative electrode include metallic materialssuch as copper, nickel, stainless steel, and nickel-plated steel. Copperis especially preferred from the standpoints of processability and cost.

In the case where the current collector is a metallic material, examplesof the shape of the current collector include metal foils, metalcylinders, metal coils, metal plates, thin metal films, expanded metals,punching metals, and metal foam. Preferred of these are thin metalfilms. More preferred are copper foils. Even more preferred are a rolledcopper foil, which is produced by the rolling process, and anelectrolytic copper foil, which is produced by the electrolytic process.Either of these can be used as a current collector.

The thickness of the current collector is generally 1 μm or more,preferably 5 μm or more, and is generally 100 μM or less, preferably 50μm or less. When the thickness of the negative-electrode currentcollector is within that range, the current collector does notconsiderably reduce the capacity of the whole battery and is easy tohandle.

(Thickness Ratio Between Current Collector and Negative-ElectrodeActive-Material Layer)

The thickness ratio between the current collector and thenegative-electrode active-material layer is not particularly limited.However, the value of “(thickness of the negative-electrodeactive-material layer on one surface just before impregnation withnonaqueous electrolytic solution)/(thickness of the current collector)”is preferably 150 or less, more preferably 20 or less, especiallypreferably 10 or less, and is preferably 0.1 or greater, more preferably0.4 or greater, especially preferably 1 or greater. When the thicknessratio between the current collector and the negative-electrodeactive-material layer is within that range, this current collector isless apt to be heated up by Joule's heat during high-current-densitycharge/discharge. Furthermore, it is easy to avoid the trouble that theproportion by volume of the current collector to the negative-electrodeactive material increases to reduce the capacity of the battery.

(Binder)

The binder for binding the negative-electrode active material is notparticularly limited so long as it is stable to the nonaqueouselectrolytic solution and to the solvent to be used for electrodeproduction.

Examples thereof include resinous polymers such as polyethylene,polypropylene, poly(ethylene terephthalate), poly(methyl methacrylate),aromatic polyamides, cellulose, and nitrocellulose; rubbery polymerssuch as SBR (styrene/butadiene rubbers), isoprene rubbers, butadienerubbers, fluororubbers, NBR (acrylonitrile/butadiene rubbers), andethylene/propylene rubbers; styrene/butadiene/styrene block copolymersor products of hydrogenation thereof; thermoplastic elastomeric polymerssuch as EPDM (ethylene/propylene/diene terpolymers),styrene/ethylene/butadiene/styrene copolymers, andstyrene/isoprene/styrene block copolymers or products of hydrogenationthereof; flexible resinous polymers such as syndiotactic1,2-polybutadiene, poly(vinyl acetate), ethylene/vinyl acetatecopolymers, and propylene/α-olefin copolymers; fluorochemical polymerssuch as poly(vinylidene fluoride), polytetrafluoroethylene, fluorinatedpoly(vinylidene fluoride), and polytetrafluoroethylene/ethylenecopolymers; and polymer compositions having the property of conductingalkali metal ions (especially lithium ions). One of these binders may beused alone, or two or more thereof may be used in any desiredcombination and proportion.

The proportion of the binder to the negative-electrode active materialis preferably 0.1% by mass or higher, more preferably 0.5% by mass orhigher, especially preferably 0.6% by mass or higher, and is preferably20% by mass or lower, more preferably 15% by mass or lower, even morepreferably 10% by mass or lower, especially preferably 8% by mass orlower. When the proportion of the binder to the negative-electrodeactive material is within that range, it is easy to avoid the troublethat the proportion of the binder which does not contribute to batterycapacity increases and this leads to a decrease in battery capacity.Furthermore, the negative electrode is inhibited from having a reducedstrength.

Especially when the binder includes a rubbery polymer represented by SBRas the main component, the proportion of this binder to thenegative-electrode active material is generally 0.1% by mass or higher,preferably 0.5% by mass or higher, more preferably 0.6% by mass orhigher, and is generally 5% by mass or lower, preferably 3% by mass orlower, more preferably 2% by mass or lower. In the case where the binderincludes a fluorochemical polymer represented by poly(vinylidenefluoride) as the main component, the proportion of this binder to thenegative-electrode active material is generally 1% by mass or higher,preferably 2% by mass or higher, more preferably 3% by mass or higher,and is generally 15% by mass or lower, preferably 10% by mass or lower,more preferably 8% by mass or lower.

(Solvent for Slurry Formation)

The kind of the solvent to be used for forming a slurry is notparticularly limited so long as the negative-electrode active materialand binder and the thickener and conductive material which areoptionally used according to need can be dissolved or dispersed therein.Either an aqueous solvent or an organic solvent may be used.

Examples of the aqueous solvent include water and alcohols. Examples ofthe organic solvent include N-methylpyrrolidone (NMP),dimethylformamide, dimethylacetamide, methyl ethyl ketone,cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine,N,N-dimethylaminopropylamine, tetrahydrofuran (THF), toluene, acetone,diethyl ether, dimethylacetamide, hexamethylphosphoramide, dimethylsulfoxide, benzene, xylene, quinoline, pyridine, methylnaphthalene, andhexane.

Especially when an aqueous solvent is used, it is preferred to add adispersant or the like in combination with a thickener and prepare aslurry using a latex of, for example, SBR. One of those solvents may beused alone, or two or more thereof may be used in any desiredcombination and proportion.

(Thickener)

A thickener is used generally for the purpose of regulating the slurryviscosity. The thickener is not particularly limited. Examples thereofinclude carboxymethyl cellulose, methyl cellulose, hydroxymethylcellulose, ethyl cellulose, poly(vinyl alcohol), oxidized starch,phosphorylated starch, casein, and salts of these. One of thesethickeners may be used alone, or two or more thereof may be used in anydesired combination and proportion.

In the case where such a thickener is further added, the proportion ofthe thickener to the negative-electrode active material is generally0.1% by mass or higher, preferably 0.5% by mass or higher, morepreferably 0.6% by mass or higher, and is generally 5% by mass or lower,preferably 3% by mass or lower, more preferably 2% by mass or lower.

When the proportion of the thickener to the negative-electrode activematerial is lower than the lower limit of that range, there are caseswhere applicability decreases considerably. When the proportion thereofis within that range, the proportion of the negative-electrode activematerial in the negative-electrode active-material layer is appropriate,and it is easy to avoid the problem that battery capacity decreases andthe trouble that resistance among the particles of thenegative-electrode active material increases.

(Electrode Density)

When the negative-electrode active material is used to form anelectrode, the electrode structure is not particularly limited. However,the density of the negative-electrode active material present on thecurrent collector is preferably 1 g·cm⁻³ or higher, more preferably 1.2g·cm⁻³ or higher, especially preferably 1.3 g·cm⁻³ or higher, and ispreferably 2 g·cm⁻³ or less, more preferably 1.9 g·cm⁻³ or less, evenmore preferably 1.8 g·cm⁻³ or less, especially preferably 1.7 g·cm⁻³ orless. When the density of the negative-electrode active material presenton the current collector is within that range, it is easy to avoid thetrouble that the negative-electrode active-material particles are brokenand this increases the initial irreversible capacity and reduces theinfiltration of a nonaqueous electrolytic solution into around thecurrent collector/negative-electrode active material interface,resulting in a deterioration in high-current-density charge/dischargecharacteristics. Furthermore, it is also possible to avoid the troublethat electrical conductivity among the negative-electrodeactive-material particles decreases and this increases batteryresistance, resulting in a decrease in capacity per unit volume.

(Thickness of Negative-Electrode Plate)

The thickness of the negative-electrode plate is designed so as to besuited for the positive-electrode plate to be used, and is notparticularly limited. However, it is desirable that the thickness of themix layer, i.e., the thickness of the negative-electrode plate excludingthe metal foil serving as a core, should be generally 15 μm or more,preferably 20 μm or more, more preferably 30 μm or more, and begenerally 150 μm or less, preferably 120 μm or less, more preferably 100μm or less.

[Positive Electrode]

The positive electrode to be used in each of the lithium secondarybatteries of the invention is explained below.

[Positive-Electrode Active Material]

The positive-electrode active material to be used in the positiveelectrode is described below.

(Composition)

The nonaqueous-electrolyte batteries of the invention are equipped witha positive-electrode active material including, as a basic composition,a lithium-containing phosphoric acid compound represented by LixMPO₄(wherein M is at least one element selected from the group consisting ofGroup-2 to Group-12 metals of the periodic table, and x satisfies0<x≦1.2).

The lithium-containing phosphoric acid compound preferably is a compoundrepresented by LixMPO₄ (wherein M is at least one element selected fromthe group consisting of the Group-4 to Group-11 transition metals in thefourth period of the periodic table, and x satisfies 0<x≦1.2).

It is preferred that M in the formula LixMPO₄ should be at least oneelement selected from the group consisting of Mg, Zn, Ca, Cd, Sr, Ba,Co, Ni, Fe, Mn, and Cu. It is more preferred that M should be at leastone element selected from the group consisting of Co, Ni, Fe, and Mn. Ofthese phosphoric acid compounds, iron lithium phosphate of an olivinestructure having the basic composition LiFePO₄ is especially suitablebecause this compound is less apt to suffer metal dissolution when in ahigh-temperature charged state and is inexpensive.

The expression “including LixMPO₄ as a basic composition” used abovemeans that not only compounds having a composition represented by theempirical formula but also compounds in which the Fe or other sites inthe crystal structure have been partly replaced by another element areincluded. Furthermore, that expression means that not only compoundshaving the stoichiometric composition but also compounds havingnon-stoichiometric compositions which include, for example, sites wherepart of the elements is deficient are included. It is preferred that theelement which replaces should be an element such as Al, Ti, V, Cr, Mn,Fe, Co, Li, Ni, Cu, Zn, Mg, Ga, Zr, or Si. In the case where replacementby such an element is conducted, the degree of replacement is preferably0.1-5 mol %, more preferably 0.2-2.5 mol %.

Although the positive-electrode active material includes the LixMPO₄ asa main component, it is possible to use this compound in combinationwith a lithium-transition metal composite oxide such as, for example, alithium-manganese composite oxide, lithium-cobalt composite oxide,lithium-nickel composite oxide, lithium-nickel-cobalt composite oxide,lithium-nickel-manganese composite oxide, orlithium-nickel-manganese-cobalt composite oxide. It is preferred thatthe positive-electrode active material includes the LixMPO₄ in an amountof 20 wt % or more. In this case, the charge/discharge cyclecharacteristics of the nonaqueous-electrolyte battery can be furtherimproved. It is more preferred that the content of the LixMPO₄ should be40 wt % or higher.

It is also possible to use two or more compounds represented by LixMPO₄in combination. Preferred examples of such combinations include:LixFePO₄ and LixMnPO₄; LixFePO₄ and LixCoPO₄; and LixFePO₄ and LixNiPO₄.Use of such a combination can improve battery operating voltage whilemaintaining safety. An especially preferred combination among these is acombination of LixFePO₄ and LixMnPO₄, because this combination bringsabout excellent durability, such as high-temperature storability andcycle characteristics, besides the improvements in safety and batteryoperating voltage.

(Surface Coating)

As a positive-electrode active material, use may be made of a materialcomposed of LixMPO₄ and, adherent to the surface thereof, a substancehaving a composition different therefrom. Examples of thesurface-adherent substance include oxides such as aluminum oxide,silicon oxide, titanium oxide, zirconium oxide, magnesium oxide, calciumoxide, boron oxide, antimony oxide, and bismuth oxide, sulfates such aslithium sulfate, sodium sulfate, potassium sulfate, magnesium sulfate,calcium sulfate, and aluminum sulfate, carbonates such as lithiumcarbonate, calcium carbonate, and magnesium carbonate, and carbon.

Those surface-adherent substances each can be adhered to the surface ofthe positive-electrode active material, for example, by: a method inwhich the substance is dissolved or suspended in a solvent and thissolution or suspension is infiltrated into the positive-electrode activematerial and then dried; a method in which a precursor for thesurface-adherent substance is dissolved or suspended in a solvent andthis solution or suspension is infiltrated into the positive-electrodeactive material and then heated or otherwise treated to react theprecursor; or a method in which the substance is added to a precursorfor the positive-electrode active material and heat-treated togetherwith the precursor. In the case where carbon is to be adhered, use maybe made of a method in which a carbonaceous substance is mechanicallyadhered later in the form of, for example, activated carbon or the like.

With respect to the amount of the surface-adherent substance to be used,the lower limit of the amount thereof, in terms of mass ppm of thepositive-electrode active material, is preferably 0.1 ppm or more, morepreferably 1 ppm or more, even more preferably 10 ppm or more. The upperlimit thereof is preferably 20% or less, more preferably 10% or less,even more preferably 5% or less, in terms of mass % based on thepositive-electrode active material. The surface-adherent substance caninhibit the electrolytic solution from undergoing an oxidation reactionon the surface of the positive-electrode active material, and animprovement in battery life can hence be attained. This effect isenhanced when the substance has been adhered in an appropriate amount.

In the invention, a material composed of a positive-electrode activematerial made of LixMPO₄ and, adherent to the surface thereof, asubstance having a composition different from the composition of theactive material is also referred to as “positive-electrode activematerial”.

(Shape)

The shape of the particles of the positive-electrode active material inthe invention may be any of massive, polyhedral, spherical, ellipsoidal,platy, acicular, columnar, and other shapes such as those in common use.Preferred of these is one in which the primary particles have aggregatedto form secondary particles and these secondary particles have aspherical or ellipsoidal shape. In electrochemical elements, the activematerial in each electrode usually expands/contracts with thecharge/discharge of the element and, hence, a deterioration, such asactive-material breakage or conduction path breakage, that is caused bythe resultant stress is apt to occur. Consequently, a positive-electrodeactive material in which the primary particles have aggregated to formsecondary particles is preferable to an active material composed ofprimary particles only since the particles in the former active materialrelieve the stress caused by expansion/contraction to prevent thedeterioration. Furthermore, particles of a spherical or ellipsoidalshape are preferable to particles showing axial orientation, e.g., platyparticles, because the former particles are less apt to orient duringelectrode forming and hence this electrode is reduced inexpansion/contraction during charge/discharge, and because theseparticles are apt to be evenly mixed with a conductive material inelectrode production.

(Tap Density)

The tap density of the positive-electrode active material is preferably0.1 g/cm³ or higher, more preferably 0.2 g/cm³ or higher, even morepreferably 0.3 g/cm³ or higher, most preferably 0.4 g/cm³ or higher. Incase where the tap density of the positive-electrode active material islower than the lower limit of that range, not only it is necessary touse a larger amount of a dispersion medium and larger amounts of aconductive material and a binder in forming a positive-electrodeactive-material layer, but also there are cases where the loading of thepositive-electrode active material in the positive-electrodeactive-material layer is limited, resulting in a limited batterycapacity. By using a composite-oxide power having a high tap density, apositive-electrode active-material layer having a high density can beformed. The higher the tap density, the more the positive-electrodeactive material is generally preferred. There is no particular upperlimit on the tap density. However, when the tap density thereof is toohigh, there are cases where the diffusion of lithium ions in thepositive-electrode active-material layer through the electrolyticsolution as a medium becomes a rate-determining stage and this is apt toreduce load characteristics. Consequently, the upper limit thereof ispreferably 2.0 g/cm³ or lower, more preferably 1.8 g/cm³ or lower.

In the invention, the tap density of a positive-electrodeactive-material powder is determined by placing 5-10 g of the powder ina 10-mL measuring cylinder made of glass, conducting a tapping operation200 times over a stroke of about 20 mm, and determining the density ofthe thus-densified powder (tap density) in terms of g/cc.

(Median Diameter d₅₀)

The median diameter d₅₀ (secondary-particle diameter in the case wherethe primary particles have aggregated to form secondary particles) ofthe particles of the positive-electrode active material is preferably0.1 μm or more, more preferably 0.2 μm or more, even more preferably 0.3μm or more, most preferably 2 μm or more. The upper limit thereof ispreferably 20 μm or less, more preferably 18 μm or less, even morepreferably 16 μm or less, most preferably 15 μm or less. When the mediandiameter d₅₀ thereof is less than the lower limit, there are cases wherea product having a high tap density cannot be obtained. In case wherethe median diameter thereof exceeds the upper limit, lithium diffusionwithin individual particles requires a longer period and this results ina decrease in battery performance. In addition, there are cases wherewhen such positive-electrode active-material particles are used inproducing a positive electrode for batteries, i.e., when the activematerial and other ingredients including a conductive material and abinder are slurried with a solvent and this slurry is applied in athin-film form, then the active material poses a problem, for example,that streak lines generate. It is possible to further improve loadingduring positive-electrode production by mixing two or morepositive-electrode active materials differing in median diameter d₅₀.

Median diameter d₅₀ in the invention is determined with a known laserdiffraction/scattering type particle size distribution analyzer. In thecase where LA-920, manufactured by HORIBA Ltd., is used as a particlesize distribution analyzer, a 0.1% by mass aqueous solution of sodiumhexametaphosphate is used as a dispersion medium for measurement toconduct a 5-minute ultrasonic dispersing treatment, before the particlesare examined at a measuring refractive index set at 1.24.

(Average Primary-Particle Diameter)

In the case where the primary particles have aggregated to formsecondary particles, the average primary-particle diameter of thispositive-electrode active material is preferably 0.02 μm or more, morepreferably 0.03 μm or more, even more preferably 0.05 μm or more. Theupper limit thereof is preferably 2 μm or less, more preferably 1.6 μmor less, even more preferably 1.3 μm or less, most preferably 1 μm orless. When the average primary-particle diameter thereof is within thatrange, spherical secondary particles are apt to be formed. It is henceeasy to avoid the trouble that the shape of secondary particlesadversely affects powder loading or results in a considerably reducedspecific surface area, resulting in a higher possibility that batteryperformance, such as output characteristics, might decrease.Furthermore, since crystal growth is insufficient, thispositive-electrode active material is less apt to pose problems such as,for example, poor charge/discharge reversibility.

Average primary-particle diameter is determined through an examinationwith a scanning electron microscope (SEM). Specifically, arbitrarilyselected 50 primary-particle images in a photograph having amagnification of 10,000 diameters each are examined for the length ofthe longest segment of a horizontal line which extends across theprimary-particle image from one side to the other side of the boundary.These measured lengths are averaged to determine the average value.

(BET Specific Surface Area)

The BET specific surface area of the positive-electrode active materialto be used in the secondary batteries of the invention is preferably 0.4m²/g or larger, more preferably 0.5 m²/g or larger, even more preferably0.6 m²/g or larger. The upper limit thereof may be 50 m²/g or smaller,preferably 40 m²/g or smaller, even more preferably 30 m²/g or smaller.When the BET specific surface area thereof is within that range, batteryperformance can be inhibited from decreasing and satisfactoryapplicability is obtained when a positive-electrode active-materiallayer is formed.

BET specific surface area is measured with a surface area meter (a fullyautomatic surface area measuring apparatus manufactured by Ohkura RikenCo., Ltd.) in the following manner. A sample is preliminarily dried at150° C. for 30 minutes in a nitrogen stream, and the specific surfacearea thereof is thereafter determined by the gas-flowing nitrogenadsorption BET one-point method using a nitrogen/helium mixture gasprecisely regulated so as to have a nitrogen pressure of 0.3 relative toatmosphere pressure. The value determined through this measurement isdefined as the BET specific surface area of the positive-electrodeactive material.

(Processes for Production)

For producing the positive-electrode active material, techniques whichare in general use as processes for producing inorganic compounds may beused. Especially for producing a spherical or ellipsoidal activematerial, various techniques may be used. Examples thereof include amethod which includes dissolving or pulverizing/dispersing a phosphorussource, e.g., phosphoric acid, and a source of the M as a component ofLixMPO₄ in a solvent, e.g., water, regulating the pH of the solution ordispersion with stirring to produce a spherical precursor, recoveringand optionally drying the precursor, subsequently adding thereto alithium source, e.g., LiOH, Li₂CO₃, or LiNO₃, and burning the mixture ata high temperature to obtain the active material.

For producing the positive electrode to be used in the invention, onepositive-electrode active material represented by LixMPO₄ and/or onepositive-electrode active material LixMPO₄ coated with thesurface-adherent substance may be used alone, or may be used togetherwith one or more such materials differing in composition in any desiredcombination or proportion. Here, the proportion of thepositive-electrode active material LixMPO₄ and/or the positive-electrodeactive material LixMPO₄ coated with the surface-adherent substance ispreferably 30% by mass or higher, more preferably 50% by mass or higher,based on all positive-electrode active materials. When the proportion ofthe positive-electrode active material LixMPO₄ and/or thepositive-electrode active material LixMPO₄ coated with thesurface-adherent substance is within that range, a preferred batterycapacity can be provided.

Incidentally, “the positive-electrode active material LixMPO₄ and/or thepositive-electrode active material LixMPO₄ coated with thesurface-adherent substance” and “positive-electrode active materialsother than the positive-electrode active material LixMPO₄ and/or thepositive-electrode active material LixMPO₄ coated with thesurface-adherent substance” are inclusively referred to as“positive-electrode active material”.

[Configuration of Positive Electrode]

The configuration of the positive electrode to be used in the inventionis described below.

(Electrode Structure and Production Process)

The positive electrode to be used in the lithium secondary batteries ofthe invention is produced by forming a positive-electrodeactive-material layer including a positive-electrode active material anda binder on a current collector. Namely, the positive electrode for thelithium secondary batteries of the invention is produced by forming apositive-electrode active-material layer including thepositive-electrode active material and a binder on a current collector.The production of the positive electrode using a positive-electrodeactive material can be conducted in an ordinary manner. Namely, apositive-electrode active material and a binder are mixed together by adry process optionally together with a conductive material, thickener,etc. and this mixture is formed into a sheet and press-bonded to apositive-electrode current collector. Alternatively, those materials aredissolved or dispersed in a liquid medium to obtain a slurry and thisslurry is applied to a positive-electrode current collector and dried.Thus, a positive-electrode active-material layer is formed on thecurrent collector, and the positive electrode can be thereby obtained.

In the positive-electrode active-material layer, the content of thepositive-electrode active material for use in the positive electrodes ofthe lithium secondary batteries of the invention is preferably 80% bymass or higher, more preferably 82% by mass or higher, especiallypreferably 84% by mass or higher. The upper limit thereof is preferably97% by mass or lower, more preferably 95% by mass or lower. When thecontent of the positive-electrode active material in thepositive-electrode active-material layer is within that range, anexcellent balance between electrical capacity and the strength of thepositive electrode is obtained.

It is preferred that the positive-electrode active-material layerobtained by coating fluid application and drying should be pressed anddensified with a handpress, roller press, or the like in order toheighten the loading density of the positive-electrode active material.The lower limit of the loading density of the positive-electrodeactive-material layer is preferably 1.3 g/cm³ or higher, more preferably1.4 g/cm³ or higher, even more preferably 1.5 g/cm³ or higher. The upperlimit thereof is preferably 3.0 g/cm³ or less, more preferably 2.5 g/cm³or less, even more preferably 2.3 g/cm³ or less.

When the density of the positive-electrode active-material layer iswithin that range, it is easy to avoid the trouble that an electrolyticsolution shows insufficient infiltration into around the currentcollector/active material interface and charge/discharge characteristicsespecially at a high current density decrease, making it impossible toobtain high output, and the trouble that electrical conductivity amongthe active-material particles decreases to increase battery resistance,making it impossible to obtain high output.

(Conductive Material)

As the conductive material, a known conductive material can be used atwill. Examples thereof include metallic materials such as copper andnickel; graphites such as natural graphites and artificial graphites;carbon blacks such as acetylene black; and carbon materials such asamorphous carbon, e.g., needle coke. One of these materials may be usedalone, or two or more thereof may be used in any desired combination andproportion. The conductive material may be used so that the material isincorporated in the positive-electrode active-material layer in anamount of generally 0.01% by mass or more, preferably 0.1% by mass ormore, more preferably 1% by mass or more, the upper limit thereof beinggenerally 50% by mass or less, preferably 30% by mass or less, morepreferably 15% by mass or less. When the content thereof is within thatrange, electrical conductivity can be sufficiently ensured and apreferred battery capacity can be provided.

(Binder)

The binder to be used for producing the positive-electrodeactive-material layer is not particularly limited. In the case where thelayer is to be formed through coating fluid application, any binder maybe used so long as it is a material which is soluble or dispersible inthe liquid medium for use in electrode production. Examples thereofinclude resinous polymers such as polyethylene, polypropylene,poly(ethylene terephthalate), poly(methyl methacrylate), polyimides,aromatic polyamides, cellulose, and nitrocellulose; rubbery polymerssuch as SBR (styrene/butadiene rubbers), NBR (acrylonitrile/butadienerubbers), fluororubbers, isoprene rubbers, butadiene rubbers, andethylene/propylene rubbers; thermoplastic elastomeric polymers such asstyrene/butadiene/styrene block copolymers or products of hydrogenationthereof, EPDM (ethylene/propylene/diene terpolymers),styrene/ethylene/butadiene/ethylene copolymers, andstyrene/isoprene/styrene block copolymers or products of hydrogenationthereof; flexible resinous polymers such as syndiotactic1,2-polybutadiene, poly(vinyl acetate), ethylene/vinyl acetatecopolymers, and propylene/α-olefin copolymers; fluorochemical polymerssuch as poly(vinylidene fluoride) (PVdF), polytetrafluoroethylene,fluorinated poly(vinylidene fluoride), andpolytetrafluoroethylene/ethylene copolymers; and polymer compositionshaving the property of conducting alkali metal ions (especially lithiumions). One of these substances may be used alone, or two or more thereofmay be used in any desired combination and proportion.

The proportion of the binder in the positive-electrode active-materiallayer is generally 0.1% by mass or higher, preferably 1% by mass orhigher, more preferably 3% by mass or higher. The upper limit thereof isgenerally 80% by mass or lower, preferably 60% by mass or lower, morepreferably 40% by mass or lower, most preferably 10% by mass or lower.When the proportion of the binder is within that range, thepositive-electrode active material can be sufficiently held and, hence,a suitable mechanical strength of the positive electrode can beprovided, making it possible to provide battery performance includingexcellent cycle characteristics, without causing a decrease in batterycapacity or electrical conductivity.

(Liquid Medium)

The kind of the liquid medium to be used for forming a slurry is notparticularly limited so long as the liquid medium is a solvent in whichthe positive-electrode active material, conductive material, and binderand a thickener, which is used according to need, can be dissolved ordispersed. Either an aqueous solvent or an organic solvent may be used.Examples of the aqueous medium include water and mixed solvents composedof an alcohol and water. Examples of the organic medium includealiphatic hydrocarbons such as hexane; aromatic hydrocarbons such asbenzene, toluene, xylene, and methylnaphthalene; heterocyclic compoundssuch as quinoline and pyridine; ketones such as acetone, methyl ethylketone, and cyclohexanone; esters such as methyl acetate and methylacrylate; amines such as diethylenetriamine andN,N-dimethylaminopropylamine; ethers such as diethyl ether, propyleneoxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone(NMP), dimethylformamide, and dimethylacetamide; and aprotic polarsolvents such as hexamethylphosphoramide and dimethyl sulfoxide.

Especially when an aqueous medium is used, it is preferred to use athickener and a latex of, for example, a styrene/butadiene rubber (SBR)to prepare a slurry. A thickener is used generally for the purpose ofregulating the viscosity of the slurry. The thickener is notparticularly limited, and examples thereof include carboxymethylcellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose,poly(vinyl alcohol), oxidized starch, phosphorylated starch, casein, andsalts of these. One of these thickeners may be used alone, or two ormore thereof may be used in any desired combination and proportion. Inthe case where such a thickener is further added, the proportion of thethickener to the active material may be 0.1% by mass or higher,preferably 0.5% by mass or higher, more preferably 0.6% by mass orhigher, and the upper limit thereof may be 5% by mass or lower,preferably 3% by mass or lower, more preferably 2% by mass or lower.When the proportion thereof is within that range, satisfactoryapplicability is obtained and the proportion of the active material inthe positive-electrode active-material layer is not excessively low. Itis therefore easy to avoid the problem that battery capacity decreasesand the trouble that resistance among particles of thepositive-electrode active material increases.

(Current Collector)

The material of the positive-electrode current collector is notparticularly limited, and a known one can be used at will. Examplesthereof include metallic materials such as aluminum, stainless steel,nickel-plated materials, titanium, and tantalum; and carbon materialssuch as carbon cloths and carbon papers. Of these, metallic materialsare preferred. Especially preferred is aluminum.

In the case of a metallic material, examples of the shape of the currentcollector include metal foils, metal cylinders, metal coils, metalplates, thin metal films, expanded metals, punching metals, and metalfoam. In the case of a carbon material, examples of the collector shapeinclude carbon plates, thin carbon films, and carbon cylinders. Ofthese, a thin metal film is preferred. The thin film may be in asuitable mesh form. Although the thin film may have any desiredthickness, the thickness thereof is generally 1 μm or more, preferably 3μm or more, more preferably 5 μm or more. The upper limit thereof isgenerally 1 mm or less, preferably 100 μm or less, more preferably 50 μmor less. When the thin film has a thickness within that range, this thinfilm can have the strength required of a current collector and is easyto handle.

Furthermore, use of a material composed of a current collector and,formed on the surface thereof, an electroconductive layer differing incompound composition from the current collector is also preferred fromthe standpoint of lowering the resistance of electronic contact betweenthe current collector and the positive-electrode active-material layer.Examples of the electroconductive layer differing in compoundcomposition from the current collector include electroconductive layersformed from carbonaceous materials, electroconductive polymers, andnoble metals such as gold, platinum, and silver.

The thickness ratio between the current collector and thepositive-electrode active-material layer is not particularly limited.However, the value of (thickness of the positive-electrodeactive-material layer on one surface just before impregnation withelectrolytic solution)/(thickness of the current collector) ispreferably 20 or less, more preferably 15 or less, most preferably 10 orless, and the lower limit thereof is preferably 0.5 or greater, morepreferably 0.8 or greater, most preferably 1 or greater. When thethickness ratio is within that range, it is easy to avoid the troublethat the current collector is heated up by Joule's heat duringhigh-current-density charge/discharge or that the proportion by volumeof the current collector to the positive-electrode active materialincreases to reduce the capacity of the battery.

(Electrode Area)

In the case where a nonaqueous electrolytic solution of the invention isused, it is preferred to regulate the positive-electrode active-materiallayer so as to have a larger area than the area of the outer surface ofthe battery case, from the standpoints of high output and enhancedhigh-temperature stability. Specifically, the total area of the positiveelectrode is preferably at least 15 times, more preferably at least 40times, the surface area of the case of the secondary battery. In thecase of a bottomed prismatic shape, the term “area of the outer surfaceof the case” means the total area calculated from the length, width, andthickness dimensions of the case part packed with the power generationelements excluding the projecting parts of the terminals. In the case ofa bottomed cylindrical shape, that term means a geometrical surface areaobtained by approximating to a cylinder the case part packed with thepower generation elements excluding the projecting parts of theterminals. The term “total area of the positive electrode” means thegeometrical surface area of the positive-electrode mix layer which facesthe mix layer containing a negative-electrode active material. In thecase of a structure obtained by forming a positive-electrode mix layeron each of both surfaces of a current collector foil, that term meansthe sum of the areas separately calculated for the respective surfaces.

(Thickness of Positive-Electrode Plate)

The thickness of the positive-electrode plate is not particularlylimited. However, from the standpoints of high capacity and high output,the thickness of the mix layer, i.e., the thickness of thepositive-electrode plate excluding the metal foil serving as a core, forone surface of the current collector is as follows. The lower limitthereof is preferably 10 μm or more, more preferably 20 μm or more, andthe upper limit thereof is preferably 500 μm or less, more preferably400 μm or less.

[Separator]

A separator is generally interposed between the positive electrode andthe negative electrode in order to prevent short-circuiting. In thiscase, a nonaqueous electrolytic solution of this invention is usuallyinfiltrated into the separator.

The material and shape of the separator are not particularly limited,and known separators can be employed at will unless the effects of theinvention are considerably lessened thereby. In particular, use may bemade of separators constituted of materials stable to the nonaqueouselectrolytic solutions of the invention, such as resins, glass fibers,and inorganic materials. It is preferred to use a separator which is inthe form of a porous sheet, nonwoven fabric, or the like and hasexcellent liquid retentivity.

As the material of the resinous or glass-fiber separators, use can bemade of, for example, polyolefins such as polyethylene andpolypropylene, polytetrafluoroethylene, polyethersulfones, glassfilters, and the like. Preferred of these are glass filters andpolyolefins. More preferred are polyolefins. One of these materials maybe used alone, or two or more thereof may be used in any desiredcombination and proportion.

The separator may have any desired thickness. However, the thicknessthereof is generally 1 μm or more, preferably 5 μM or more, morepreferably 10 μm or more, and is generally 50 μm or less, preferably 40μm or less, more preferably 30 μm or less. When the thickness of theseparator is within that range, neither insulating properties normechanical strength decreases, and the battery is less apt to suffer adecrease in battery performance, e.g., rate characteristics, or adecrease in the energy density of the nonaqueous-electrolyte battery asa whole.

In the case where a porous material such as, e.g., a porous sheet ornonwoven fabric is used as the separator, this separator may have anydesired porosity. However, the porosity thereof is generally 20% orhigher, preferably 35% or higher, more preferably 45% or higher, and isgenerally 90% or lower, preferably 85% or lower, more preferably 75% orlower. When the porosity thereof is within that range, this separatordoes not have excessively high film resistance, and preferred ratecharacteristics can be provided. Furthermore, this separator does nothave reduced mechanical strength, and preferred insulating propertiesalso can be provided.

The separator may have any desired average pore diameter. However, theaverage pore diameter thereof is generally 0.5 μm or less, preferably0.2 μm or less, and is generally 0.05 μm or more. When the average porediameter thereof is within that range, short-circuiting is less apt tooccur, and this separator does not have excessively high film resistanceand can provide preferred rate characteristics.

On the other hand, examples of the inorganic materials which may be usedinclude oxides such as alumina and silicon dioxide, nitrides such asaluminum nitride and silicon nitride, and sulfates such as bariumsulfate and calcium sulfate. Such materials of a particulate shape orfibrous shape may be used.

With respect to form, a separator of a thin film form may be used, suchas nonwoven fabric, woven fabric, or microporous film. Suitableseparators of a thin film form have a pore diameter of 0.01-1 μm and athickness of 5-50 μm. Besides such a separator in an independent thinfilm form, use can be made of a separator obtained by forming acomposite porous layer containing particles of the inorganic materialusing a resinous binder on a surface layer of the positive electrodeand/or negative electrode. Examples thereof include to form a porouslayer from alumina particles having a 90% particle diameter smaller than1 μm on both surfaces of the positive electrode using a fluororesin as abinder.

[Battery Design]

<Electrode Group>

The electrode group may be either of: an electrode group having amultilayer structure in which the positive-electrode plate andnegative-electrode plate have been superposed through the separator; andan electrode group having a wound structure in which thepositive-electrode plate and negative-electrode plate have been spirallywound through the separator. The proportion of the volume of theelectrode group to the internal volume of the battery (hereinafterreferred to as electrode group proportion) is generally 40% or higher,preferably 50% or higher, and is generally 90% or lower, preferably 80%or lower.

When the electrode group proportion is within that range, not only apreferred battery capacity can be provided, but also a moderate spacevolume can be ensured. Consequently, this battery does not undergo anexcessive increase in internal pressure, even when the battery is heatedup to cause members to expand or a liquid component of the electrolyteto have a heightened vapor pressure. It is therefore easy to avoid thetrouble that the battery is reduced in various characteristics includingcharge/discharge cycling performance and high-temperature storability,and the trouble that gas release valve, which releases the internalpressure, works.

<Structure for Current Collection>

The structure for current collection is not particularly limited.However, for more effectively attaining the improvement in high-currentdensity charge/discharge characteristics which is brought about by thenonaqueous electrolytic solutions of this invention, it is preferred toemploy a structure reduced in the resistance of wiring parts and jointparts. In the case where internal resistance has been reduced in thismanner, use of the nonaqueous electrolytic solutions of the inventionproduces the effects thereof especially satisfactorily.

In the case of an electrode group assembled into the multilayerstructure described above, a structure obtained by bundling the metalliccore parts of respective electrode layers and welding the bundled partsto a terminal is suitable. When each electrode has a large area, thisresults in increased internal resistance. In this case, it is preferredto dispose a plurality of terminals in each electrode to reduce theresistance. In the case of an electrode group having the wound structuredescribed above, a plurality of lead structures may be disposed on eachof the positive electrode and negative electrode and bundled into aterminal. Thus, internal resistance can be reduced.

<Protective Element>

Examples of the protective element include a PTC (positive temperaturecoefficient), which increases in resistance upon abnormal heating-up orwhen an excessive current flows, a temperature fuse, a thermister, and avalve (current breaker valve) which, upon abnormal heating-up, breaksthe current flowing through the circuit, on the basis of an abruptincrease in the internal pressure or internal temperature of thebattery. It is preferred to select such a protective element which doesnot work under ordinary high-current use conditions. It is morepreferred to employ a design which prevents abnormal heating-up andthermal run-away even without a protective element.

<Case>

The nonaqueous-electrolyte batteries of the invention each are usuallyfabricated by housing the nonaqueous electrolytic solution, negativeelectrode, positive electrode, separator, etc. in a case. This case isnot particularly limited, and a known case can be employed at willunless this considerably lessens the effects of the invention.Specifically, although the case may be made of any desired material, useis generally made of a metal such as nickel-plated iron, stainlesssteel, aluminum or an alloy thereof, nickel, titanium, or a magnesiumalloy or a laminated film composed of a resin and an aluminum foil. Fromthe standpoint of weight reduction, a metal which is aluminum or analuminum alloy or a laminated film is suitable.

Examples of the case obtained using any of those metals include: a caseformed by fusion-bonding a metal to itself by laser welding, resistancewelding, or ultrasonic welding to constitute a sealed structure; or acase formed by caulking any of those metals through a resinous gasket.Examples of the case obtained using the laminated film include a casehaving a sealed structure formed by thermally fusion-bonding the resinlayer to itself. A resin different from the resin used in the laminatedfilm may be interposed between the resin layers in order to enhancesealing properties. Especially when a sealed structure is to be formedby thermally fusion-bonding resin layers through current-collectorterminals, then metal/resin bonding is involved and, hence, a resinhaving polar groups or a modified resin into which polar groups havebeen incorporated is suitable for use as the resin to be interposed.

The case may have any desired shape. For example, the case may be any ofthe cylindrical type, prismatic type, laminate type, coin type, largetype, and the like.

EXAMPLES

The invention will be explained below in more detail by reference toExamples and Comparative Examples. However, the invention should not beconstrued as being limited to the following Examples unless theinvention departs from the spirit thereof.

Example 1 Production of Negative Electrode

To 98 parts by weight of artificial-graphite powder KS-44 (trade name;manufactured by Timcal) were added 100 parts by weight of an aqueousdispersion of sodium carboxymethyl cellulose (concentration of sodiumcarboxymethyl cellulose, 1% by mass) as a thickener and 2 parts byweight of an aqueous dispersion of a styrene/butadiene rubber(concentration of styrene/butadiene rubber, 50% by mass) as a binder.The ingredients were mixed together by means of a disperser to obtain aslurry. The slurry obtained was applied to one surface of a copper foilhaving a thickness of 10 μm and dried. This coated foil was rolled witha pressing machine to a thickness of 75 μm, and a piece of a shapehaving an active-material layer size with a width of 30 mm and a lengthof 40 mm and having an uncoated area with a width of 5 mm and a lengthof 9 mm was cut out of the rolled sheet. Thus, a negative electrode wasobtained.

[Production of Positive Electrode]

Ninety percents by mass iron lithium phosphate (LiFePO₄; manufactured bySTL Energy Technology Co., Ltd.) as a positive-electrode active materialwas mixed with 5% by mass acetylene black as a conductive material and5% by mass poly(vinylidene fluoride) (PVdF) as a binder inN-methylpyrrolidone solvent to obtain a slurry. The slurry obtained wasapplied to one surface of an aluminum foil having a thickness of 15 μmand coated beforehand with a carbonaceous material, and dried. Thiscoated foil was rolled with a pressing machine to a thickness of 80 μm,and a piece of a shape having an active-material layer size with a widthof 30 mm and a length of 40 mm and having a uncoated area with a widthof 5 mm and a length of 9 mm was cut out of the rolled sheet. Thus, apositive electrode was obtained.

[Production of Electrolytic Solution]

In a dry argon atmosphere, 98.5% by mass mixture of ethylene carbonate(EC), dimethoxyethane (DME), and ethyl methyl carbonate (EMC) (volumeratio, 2:3:5) was mixed with 0.5% by mass lithium difluorophosphate(LiPO₂F₂), 0.5% by mass vinylene carbonate (VC), and 0.5% by massmonofluoroethylene carbonate (MFEC). Subsequently, sufficiently driedLiPF₆ was dissolved therein so as to result in a proportion thereof of1.1 mol/L. Thus, an electrolytic solution was obtained.

[Production of Lithium Secondary Battery]

The positive electrode and negative electrode described above and aseparator made of polyethylene were superposed in the order of negativeelectrode/separator/positive electrode to produce a battery element.This battery element was inserted into a bag constituted of a laminatedfilm obtained by coating both surfaces of aluminum (thickness, 40 μm)with a resin layer, with the terminals of the positive and negativeelectrodes projecting outward. Thereafter, the electrolytic solution wasintroduced into the bag, and this bag was vacuum-sealed to produce asheet battery. This battery was evaluated. The components of theelectrolytic solution are shown in Table 1.

[Evaluation of Initial Discharge Capacity]

The lithium secondary battery was evaluated in the state of beingsandwiched between glass plates in order to enhance contact between theelectrodes. At 25° C., this battery was charged to 4.0 V at a constantcurrent corresponding to 0.2 C and then discharged to 2.5 V at aconstant current of 0.1 C. Two cycles of this charge/discharge wereconducted to stabilize the battery. In each of the third to sixthcycles, the battery was charged to 4.0 V at a constant current of 0.2 C,subsequently charged at a constant voltage of 4.0 V until the currentvalue became 0.05 C, and then discharged to 2.5 V at a constant currentof 0.2 C. Thereafter, in the seventh cycle, the battery was charged to4.0 V at a constant current of 0.2 C, subsequently charged at a constantvoltage of 4.0 V until the current value became 0.05 C, and thendischarged to 2.5 V at a constant current of 0.2 C to determine initialdischarge capacity. Here, “1 C” means a current value at which thereference capacity of the battery is discharged over 1 hour; “5 C” meansthe current value which is 5 times the current of 1 C, “0.1 C” means thecurrent value which is 1/10 the current of 1 C, and “0.2 C” means thecurrent value which is ⅕ the current of 1 C.

[Evaluation of 25° C. Output]

The battery which had undergone the evaluation of initial dischargecapacity was charged at 25° C. and a constant current of 0.2 C to a halfof the initial discharge capacity. This battery was discharged at 25° C.for 10 seconds at each of 1 C, 2 C, 4 C, 7 C, 10 C, and 15 C, and thevoltage was measured at the time when the 10 seconds had passed. Thearea of the triangle surrounded by the current-voltage line and thelower-limit voltage (2.5 V) was regarded as output (W). The relativevalue (%) of the output was calculated, with the output value at 25° C.of Comparative Example 1 being taken as 100.

[Evaluation of −30° C. Output]

The battery which had undergone the evaluation of initial dischargecapacity was charged at 25° C. and a constant current of 0.2 C to a halfof the initial discharge capacity. This battery was discharged at −30°C. for 10 seconds at each of 0.2 C, 0.4 C, 0.8 C, 1 C, and 2 C, and thevoltage was measured at the time when the 10 seconds had passed. Thearea of the triangle surrounded by the current-voltage line and thelower-limit voltage (2.5 V) was regarded as output (W). The relativevalue (%) of the output was calculated, with the output value at −30° C.of Comparative Example 1 being taken as 100.

[Evaluation of High-Temperature Cycle Characteristics]

At 60° C., the battery which had undergone the test for evaluatinginitial discharge capacity was charged to 3.6 V at a constant current of2 C and then discharged to 2.5 V at a constant current of 2 C. Thisoperation was taken as one cycle, and 500 cycles were conducted. Thedischarge capacity (%) in the 500th cycle was determined as a valuerelative to the discharge capacity measured in the first cycle, whichwas taken as 100. This discharge capacity (%) was taken as dischargecapacity retention.

[Evaluation of High-Rate Discharge Characteristics]

The battery which had undergone the high-temperature cycle test wassubjected to the following test at 25° C. The battery was charged to 3.6V at a constant current of 0.2 C and then charged at a constant voltageof 3.6 V until the current value became 0.05 C. This battery wasdischarged to 2.5 V at each of constant currents of 2 C and 5 C. Thedischarge capacities (%) at 2 C and 5 C after the high-temperature cycletest were determined as values relative to the discharge capacitydetermined in the initial discharge capacity test, which was taken as100.

The results of the evaluation are shown in Tables 2 and 3.

Example 2

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.5% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 2:3:5) with 0.5% by mass vinylenecarbonate (VC) and then dissolving sufficiently dried LiPF₆ in theresultant mixture so as to result in a proportion thereof of 1.1 mol/L.The components of the electrolytic solution and the results of theevaluation are shown in Table 1 to Table 3.

Example 3

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.5% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 2:3:5) with 0.5% by mass lithiumdifluorophosphate (LiPO₂F₂) and then dissolving sufficiently dried LiPF₆in the resultant mixture so as to result in a proportion thereof of 1.1mol/L. The components of the electrolytic solution and the results ofthe evaluation are shown in Table 1 to Table 3.

Example 4

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 98.5% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 3:2:5) with 0.5% by mass lithiumdifluorophosphate (LiPO₂F₂), 0.5% by mass vinylene carbonate (VC), and0.5% by mass monofluoroethylene carbonate (MFEC) and then dissolvingsufficiently dried LiPF₆ in the resultant mixture so as to result in aproportion thereof of 1.1 mol/L. The components of the electrolyticsolution and the results of the evaluation are shown in Table 1 to Table3.

Example 5

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.5% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 3:2:5) with 0.5% by mass vinylenecarbonate (VC) and then dissolving sufficiently dried LiPF₆ in theresultant mixture so as to result in a proportion thereof of 1.1 mol/L.The components of the electrolytic solution and the results of theevaluation are shown in Table 1 to Table 3.

Example 6

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.0% by mass mixture ofethylene carbonate (EC), diethoxyethane (DEE), and ethyl methylcarbonate (EMC) (volume ratio, 3:1:6) with 0.5% by mass vinylenecarbonate (VC) and 0.5% by mass lithium difluorophosphate (LiPO₂F₂) andthen dissolving sufficiently dried LiPF₆ in the resultant mixture so asto result in a proportion thereof of 1.1 mol/L. The components of theelectrolytic solution and the results of the evaluation are shown inTable 1 to Table 3.

Example 7

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.0% by mass mixture ofethylene carbonate (EC), ethoxy(2,2,2-trifluoroethoxy)ethane (ETFEE),and ethyl methyl carbonate (EMC) (volume ratio, 3:1:6) with 0.5% by massvinylene carbonate (VC) and 0.5% by mass lithium difluorophosphate(LiPO₂F₂) and then dissolving sufficiently dried LiPF₆ in the resultantmixture so as to result in a proportion thereof of 1.1 mol/L. Thecomponents of the electrolytic solution and the results of theevaluation are shown in Table 1 to Table 3.

Example 8

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.0% by mass mixture ofethylene carbonate (EC), 1,1,2,2-tetrafluoroethyl2,2,3,3-tetrafluoro-n-propyl ether (TFETFPE), and ethyl methyl carbonate(EMC) (volume ratio, 3:1:6) with 0.5% by mass vinylene carbonate (VC)and 0.5% by mass lithium difluorophosphate (LiPO₂F₂) and then dissolvingsufficiently dried LiPF₆ in the resultant mixture so as to result in aproportion thereof of 1.1 mol/L. The components of the electrolyticsolution and the results of the evaluation are shown in Table 1 to Table3.

Example 9

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 98.5% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 3:2:5) with 0.5% by mass vinylenecarbonate (VC) and 1% by mass lithium trifluoromethanesulfonate(CF₃SO₃Li) and then dissolving sufficiently dried LiPF₆ in the resultantmixture so as to result in a proportion thereof of 1.1 mol/L. Thecomponents of the electrolytic solution and the results of theevaluation are shown in Table 1 to Table 3.

Example 10

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 98.5% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 3:2:5) with 0.5% by mass vinylenecarbonate (VC) and 1% by mass lithium bis(fluorosulfonyl)imide (LiFSI)and then dissolving sufficiently dried LiPF₆ in the resultant mixture soas to result in a proportion thereof of 1.1 mol/L. The components of theelectrolytic solution and the results of the evaluation are shown inTable 1 to Table 3.

Example 11

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.0% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 3:2:5) with 0.3% by mass vinylenecarbonate (VC), 0.2% by mass 1,3-propanesultone (PS), and 0.5% by masslithium difluorophosphate (LiPO₂F₂) and then dissolving sufficientlydried LiPF₆ in the resultant mixture so as to result in a proportionthereof of 1.1 mol/L. The components of the electrolytic solution andthe results of the evaluation are shown in Table 1 to Table 3.

Example 12

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.0% by mass mixture ofethylene carbonate (EC), dimethoxyethane (DME), and ethyl methylcarbonate (EMC) (volume ratio, 3:2:5) with 0.3% by mass vinylenecarbonate (VC), 0.2% by mass ethylene sulfite (ES), and 0.5% by masslithium difluorophosphate (LiPO₂F₂) and then dissolving sufficientlydried LiPF₆ in the resultant mixture so as to result in a proportionthereof of 1.1 mol/L. The components of the electrolytic solution andthe results of the evaluation are shown in Table 1 to Table 3.

Comparative Example 1

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.5% by mass mixture ofethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methylcarbonate (EMC) (volume ratio, 3:3:4) with 0.5% by mass vinylenecarbonate (VC) and then dissolving sufficiently dried LiPF₆ in theresultant mixture so as to result in a proportion thereof of 1 mol/L.The components of the electrolytic solution and the results of theevaluation are shown in Table 1 to Table 3.

Comparative Example 2

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99% by mass mixture ofethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methylcarbonate (EMC) (volume ratio, 3:3:4) with 1% by mass vinylene carbonate(VC) and then dissolving sufficiently dried LiPF₆ in the resultantmixture so as to result in a proportion thereof of 1 mol/L. Thecomponents of the electrolytic solution and the results of theevaluation are shown in Table 1 to Table 3.

Comparative Example 3

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.5% by mass mixture ofethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methylcarbonate (EMC) (volume ratio, 3:3:4) with 0.5% by mass vinylethylenecarbonate (VEC) and then dissolving sufficiently dried LiPF₆ in theresultant mixture so as to result in a proportion thereof of 1 mol/L.The components of the electrolytic solution and the results of theevaluation are shown in Table 1 to Table 3.

Comparative Example 4

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing 99.5% by mass mixture ofethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methylcarbonate (EMC) (volume ratio, 3:3:4) with 0.5% by mass1,3-propanesultone (PS) and then dissolving sufficiently dried LiPF₆ inthe resultant mixture so as to result in a proportion thereof of 1mol/L. The components of the electrolytic solution and the results ofthe evaluation are shown in Table 1 to Table 3.

Comparative Example 5

A sheet-form lithium secondary battery was produced and evaluated in thesame manners as in Example 1, except that an electrolytic solution wasobtained in a dry argon atmosphere by mixing a mixture of ethylenecarbonate (EC), dimethoxyethane (DME), and ethyl methyl carbonate (EMC)(volume ratio, 3:2:5) and then dissolving sufficiently dried LiPF₆ inthe resultant mixture so as to result in a proportion thereof of 1.1mol/L. The components of the electrolytic solution and the results ofthe evaluation are shown in Table 1 to Table 3.

Comparative Example 6 Production of Positive Electrode

Ninety percents by mass Li(Ni_(1/3)Mn_(1/3)Co_(1/3))O₂ (NMC) as apositive-electrode active material was mixed with 5% by mass acetyleneblack as a conductive material and 5% by mass poly(vinylidene fluoride)(PVdF) as a binder in N-methylpyrrolidone solvent to obtain a slurry.The slurry obtained was applied to one surface of an aluminum foilhaving a thickness of 15 μm and coated beforehand with a conduction aid,and dried. This coated foil was rolled with a pressing machine to athickness of 80 μm, and a piece of a shape having an active-materiallayer size with a width of 30 mm and a length of 40 mm and having auncoated area with a width of 5 mm and a length of 9 mm was cut out ofthe rolled sheet. Thus, a positive electrode was obtained.

[Production of Electrolytic Solution]

A sheet-form lithium secondary battery was produced in the same manneras in Example 1, except that an electrolytic solution was obtained in adry argon atmosphere by mixing 99.0% by mass mixture of ethylenecarbonate (EC), dimethoxyethane (DME), and ethyl methyl carbonate (EMC)(volume ratio, 2:3:5) with 1.0% by mass LiN(FSO₂)₂ (LiFSI) and thendissolving sufficiently dried LiPF₆ in the resultant mixture so as toresult in a proportion thereof of 1.1 mol/L. The components of theelectrolytic solution are shown in Table 1.

[Evaluation of Initial Discharge Capacity]

The lithium secondary battery produced above was evaluated in the stateof being sandwiched between glass plates in order to enhance contactbetween the electrodes. At 25° C., this battery was charged to 4.1 V ata constant current corresponding to 0.2 C and then discharged to 3 V ata constant current of 0.2 C. Two cycles of this charge/discharge wereconducted to stabilize the battery. In the third cycle, the battery wascharged to 4.2 V at a constant current of 0.2 C, subsequently charged ata constant voltage of 4.2 V until the current value became 0.05 C, andthen discharged to 3 V at a constant current of 0.2 C. Thereafter, inthe fourth cycle, the battery was charged to 4.2 V at a constant currentof 0.2 C, subsequently charged at a constant voltage of 4.2 V until thecurrent value became 0.05 C, and then discharged to 3 V at a constantcurrent of 0.2 C to determine initial discharge capacity. Here, “1 C”means a current value at which the reference capacity of the battery isdischarged over 1 hour; “5 C” means the current value which is 5 timesthe current of 1 C, “0.1 C” means the current value which is 1/10 thecurrent of 1 C, and “0.2 C” means the current value which is ⅕ thecurrent of 1 C.

[Evaluation of High-Temperature Cycle Characteristics]

At 60° C., the battery which had undergone the test for evaluatinginitial discharge capacity was charged to 4.2 V at a constant current of2 C and then discharged to 3 V at a constant current of 2 C. Thisoperation was taken as one cycle, and 500 cycles were conducted. Thedischarge capacity (%) in the 500th cycle was determined as a valuerelative to the discharge capacity measured in the first cycle, whichwas taken as 100. This discharge capacity (%) was taken as dischargecapacity retention.

[Evaluation of High-Rate Discharge Characteristics]

The battery which had undergone the high-temperature cycle test wassubjected to the following test at 25° C. The battery was charged to 4.2V at a constant current of 0.2 C and then charged at a constant voltageof 4.2 V until the current value became 0.05 C. This battery wasdischarged to 3 V at each of constant currents of 2 C and 5 C. Thedischarge capacities (%) at 2 C and 5 C after the high-temperature cycletest were determined as values relative to the discharge capacitydetermined in the initial discharge capacity test, which was taken as100.

The results of the evaluation are shown in Tables 2 and 3.

TABLE 1 Solvent (ratio) Additive (mass %) Example 1 EC:DME:EMC LiPO₂F₂(0.5) (20:30:50) VC (0.5) MFEC (0.5) Example 2 EC:DME:EMC VC (0.5)(20:30:50) Example 3 EC:DME:EMC LiPO₂F₂ (0.5) (20:30:50) Example 4EC:DME:EMC LiPO₂F₂ (0.5) (30:20:50) VC (0.5) MFEC (0.5) Example 5EC:DME:EMC VC (0.5) (30:20:50) Example 6 EC:DEE:EMC VC (0.5) (30:10:60)LiPO₂F₂ (0.5) Example 7 EC:ETFEE:EMC VC (0.5) (30:10:60) LiPO₂F₂ (0.5)Example 8 EC:TFETFPE:EMC VC (0.5) (30:10:60) LiPO₂F₂ (0.5) Example 9EC:DME:EMC VC (0.5) (30:20:50) CF₃SO₃Li (1) Example 10 EC:DME:EMC VC(0.5) (30:20:50) LiFSI (1) Example 11 EC:DME:EMC VC (0.3) (30:20:50) PS(0.2) LiPO₂F₂ (0.5) Example 12 EC:DME:EMC VC (0.3) (30:20:50) ES (0.2)LiPO₂F₂ (0.5) Comparative EC:DMC:EMC VC (0.5) Example 1 (30:30:40)Comparative EC:DMC:EMC VC (1.0) Example 2 (30:30:40) ComparativeEC:DMC:EMC VEC (0.5) Example 3 (30:30:40) Comparative EC:DMC:EMC PS(0.5) Example 4 (20:30:50) Comparative EC:DME:EMC none Example 5(20:30:50) Comparative EC:DME:EMC LiFSI (1) Example 6 (20:30:50)

TABLE 2 Output relative to Comparative Example 1 (%) 25° C. −30° C.Example 1 115.5 149.3 Example 2 112.2 114.0 Example 3 116.6 176.6Example 4 105.7 118.8 Example 5 108.0 105.9 Example 6 111.3 133.3Example 7 110.4 128.5 Example 8 107.8 122.0 Example 9 112.0 136.5Example 10 112.6 134.9 Example 11 111.0 130.1 Example 12 113.0 131.7Comparative 100 100 Example 1 Comparative 98.5 93.1 Example 2Comparative 91.5 94.7 Example 3 Comparative 90.8 99.5 Example 4

TABLE 3 Discharge capacity 2-C discharge 5-C discharge retention aftercapacity after capacity after 500 cycles (%) 500 cycles (%) 500 cycles(%) Example 1 66.4 59.9 58.7 Example 2 65.1 59.3 57.6 Example 3 51.439.1 37.9 Example 4 77.8 72.4 69.5 Example 6 71.3 64.8 61.5 Example 765.4 60.9 57.4 Example 8 71.7 62.1 58.6 Example 9 67.8 66.5 63.9 Example10 68.1 65.6 62.9 Example 11 64.8 58.7 55.9 Example 12 67.8 60.0 56.9Comparative 23.4 26.1 23.5 Example 4 Comparative 40.7 33.9 29.7 Example5 Comparative 45.0 35.7 30.5 Example 6

As apparent from Table 2, the nonaqueous-electrolyte batteries of theinvention are superior in initial output at 25° C. and −30° C. Asapparent from Table 3, the batteries of the invention are superior inhigh-temperature cycle characteristics and in high-current-densitydischarge characteristics determined after the high-temperature cycletest. It was hence found that the batteries of the invention have highdurability. In contrast, the batteries employing nonaqueous electrolyticsolutions which are not the nonaqueous electrolytic solutions accordingto the invention are lower in initial output at 25° C. and −30° C. thanthe batteries employing the nonaqueous electrolytic solutions accordingto the invention, and are inferior in high-temperature cyclecharacteristics and in high-current-density discharge characteristicsdetermined after the high-temperature cycle test.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof. This application is basedon a Japanese patent application filed on Sep. 29, 2009 (Application No.2009-223809), the contents thereof being incorporated herein byreference.

INDUSTRIAL APPLICABILITY

The nonaqueous-electrolyte batteries of the invention have a highinitial output at ordinary temperature and −30° C., attain a highdischarge capacity even during high-rate discharge, and have a highcapacity retention after a durability test such as a high-temperaturestorage test or cycle test. In addition, even after the durability test,the batteries of the invention have the excellent output performance andhigh-rate discharge capacity equal to the initial values.

1. A nonaqueous-electrolyte battery which comprises: a currentcollector; a positive electrode containing a lithium-containingphosphoric acid compound represented by LixMPO₄ (wherein M is at leastone element selected from the group consisting of Group-2 to Group-12metals of the periodic table, and x satisfies 0<x≦1.2) as apositive-electrode active material; a negative electrode containing anegative-electrode active material capable of occluding and releasinglithium ions; and a nonaqueous electrolytic solution, wherein thenonaqueous electrolytic solution contains (1) a chain ether and (2) acyclic carbonate having an unsaturated bond.
 2. A nonaqueous-electrolytebattery which comprises: a current collector; a positive electrodecontaining a lithium-containing phosphoric acid compound represented byLixMPO₄ (wherein M is at least one element selected from the groupconsisting of Group-2 to Group-12 metals of the periodic table, and xsatisfies 0<x≦1.2) as a positive-electrode active material; a negativeelectrode containing a negative-electrode active material capable ofoccluding and releasing lithium ions; and a nonaqueous electrolyticsolution, wherein the nonaqueous electrolytic solution contains (1) achain ether and (2) at least one compound selected from lithiumfluorophosphates, lithium sulfonates, imide lithium salts, sulfonic acidesters, and sulfurous acid esters.
 3. The nonaqueous-electrolyte batteryaccording to claim 1 or 2, wherein the lithium-containing phosphoricacid compound is represented by LixMPO₄ (wherein M is at least oneelement selected from the group consisting of the Group-4 to Group-11transition metals in the fourth period of the periodic table, and xsatisfies 0<x≦1.2).
 4. The nonaqueous-electrolyte battery according toclaim 1, wherein the content of the cyclic carbonate having anunsaturated bond is 0.001-5% by mass based on the whole electrolyticsolution.
 5. The nonaqueous-electrolyte battery according to claim 1 or2, wherein the nonaqueous electrolytic solution contains ethylenecarbonate in an amount of 10% by volume or more.
 6. Thenonaqueous-electrolyte battery according to claim 1 or 2, wherein thechain ether is represented by R¹OR² (wherein R¹ and R² each represent amonovalent organic group which has 1-8 carbon atoms and may have afluorine atom, and R¹ and R² may be the same or different).
 7. Thenonaqueous-electrolyte battery according to claim 1 or 2, wherein thenegative-electrode active material is a carbonaceous material.
 8. Thenonaqueous-electrolyte battery according to claim 1 or 2, wherein thecurrent collector has an electroconductive layer on the surface thereof,the electroconductive layer being different from the current collectorin compound composition.
 9. A nonaqueous electrolytic solution for usein the nonaqueous-electrolyte battery according to claim 1 or 2.